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OK, here we go with,
quiz review for the third quiz

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that's coming on Friday.

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So, one key point is that the
quiz covers through chapter six.

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Chapter seven on linear
transformations will appear on

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the final exam,
but not on the quiz.

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So I won't review linear
transformations today,

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but they'll come into the full
course review on the very last

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lecture.

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So today, I'm reviewing chapter
six, and I'm going to take some

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old exams, and I'm always ready
to answer questions.

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And I thought,
kind of help our memories if I

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write down the main topics in
chapter six.

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So, already,
on the previous quiz,

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we knew how to find eigenvalues
and eigenvectors.

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Well, we knew how to find them
by that determinant of A minus

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lambda I equals zero.

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But, of course,
there could be shortcuts.

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There could be,
like, useful information about

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the eigenvalues that we can
speed things up with.

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OK.

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Then, the new stuff starts out
with a differential equation,

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so I'll do a problem.

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I'll do a differential equation
problem first.

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What's special about symmetric
matrices?

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Can we just say that in words?

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I'd better write it down,
though.

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What's special about symmetric
matrices?

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Their eigenvalues are real.

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The eigenvalues of a symmetric
matrix always come out real,

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and there always are enough
eigenvectors.

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Even if there are repeated
eigenvalues, there are enough

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eigenvectors,
and we can choose those

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eigenvectors to be orthogonal.

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So if A equals A transposed,
the big fact will be that we

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can diagonalize it,
and those eigenvector matrix,

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with the eigenvectors in the
column, can be an orthogonal

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matrix.

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So we get a Q lambda Q
transpose.

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That, in three symbols,
expresses a wonderful fact,

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a fundamental fact for
symmetric matrices.

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OK.

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Then, we went beyond that fact
to ask about positive definite

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matrices, when the eigenvalues
were positive.

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I'll do an example of that.

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Now we've left symmetry.

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Similar matrices are any square
matrices, but two matrices are

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similar if they're related that
way.

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And what's the key point about
similar matrices?

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Somehow, those matrices are
representing the same thing in

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different basis,
in chapter seven language.

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In chapter six language,
what's up with these similar

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matrices?

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What's the key fact,
the key positive fact about

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similar matrices?

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They have the same eigenvalues.

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Same eigenvalues.

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So if one of them grows,
the other one grows.

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If one of them decays to zero,
the other one decays to zero.

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Powers of A will look like
powers of B, because powers of A

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and powers of B only differ by
an M inverse and an M way on the

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outside.

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So if these are similar,
then B to the k-th power is M

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inverse A to the k-th power M.

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And that's why I say,
eh, this M, it does change the

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eigenvectors,
but it doesn't change the

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eigenvalues.

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So same lambdas.

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And then, finally,
I've got to review the point

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about the SVD,
the Singular Value

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Decomposition.

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OK.

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So that's what this quiz has
got to cover,

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and now I'll just take problems
from earlier exams,

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starting with a differential
equation.

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OK.

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And always ready for questions.

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So here is an exam from about
the year zero,

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and it has a three by three.

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So that was -- but it's a
pretty special-looking matrix,

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it's got zeroes on the
diagonal, it's got minus ones

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above, and it's got plus ones
like that.

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So that's the matrix A.

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OK.

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Step one is,
well, I want to solve that

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equation.

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I want to find the general
solution.

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I haven't given you a u(0)
here, so I'm looking for the

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general solution,
so now what's the form of the

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general solution?

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With three arbitrary constants
going to be inside it,

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because those will be used to
match the initial condition.

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So the general form is u at
time t is some multiple of the

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first special solution.

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The first special solution will
be growing like the eigenvalue,

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and it's the eigenvector.

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So that's a pure exponential
solution, just staying with that

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eigenvector.

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Of course, I haven't found,
yet, the eigenvalues and

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eigenvectors.

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That's, normally,
the first job.

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Now, there will be second one,
growing like e to the lambda

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two, and a third one growing
like e to the lambda three.

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So we're all done -- well,
we haven't done anything yet,

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actually.

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I've got to find the
eigenvalues and eigenvectors,

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and then I would match u(0) by
choosing the right three

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constants.

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OK.

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So now I ask -- ask you about
the eigenvalues and

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eigenvectors,
and you look at this matrix and

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what do you see in that matrix?

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Um, well, I guess we might ask
ourselves right away,

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is it singular?

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Is it singular?

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Because, if so,
then we really have a head

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start, we know one of the
eigenvalues is zero.

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Is that matrix singular?

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Eh, I don't know,
do you take the determinant to

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find out?

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Or maybe you look at the first
row and third row and say,

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hey, the first row and third
row are just opposite signs,

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they're linear-dependent?

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The first column and third
column are dependent -- it's

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singular.

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So one eigenvalue is zero.

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Let's make that lambda one.

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Lambda one, then,
will be zero.

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OK.

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Now we've got a couple of other
eigenvalues to find,

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and, I suppose the simplest way
is to look at A minus lambda I

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So let me just put minus lambda
in here, minus ones above,

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ones below.

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But, actually,
before I do it,

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that matrix is not symmetric,
for sure, right?

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In fact, it's the very opposite
of symmetric.

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That matrix A transpose,
how is A transpose connected to

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A?

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It's negative A.

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It's an anti-symmetric matrix,
skew-symmetric matrix.

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And we've met,
maybe, a two-by-two example of

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skew-symmetric matrices,
and let me just say,

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what's the deal with their
eigenvalues?

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They're pure imaginary.

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They'll be on the imaginary
axis, there be some multiple of

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I if it's an anti-symmetric,
skew-symmetric matrix.

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So I'm looking for multiples of
I, and of course,

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that's zero times I,
that's on the imaginary axis,

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but maybe I just do it out,
here.

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Lambda cubed.
well, maybe that's minus lambda

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cubed, and then a zero and a
zero.

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Zero, and then maybe I have a
plus a lambda,

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and another plus lambda,
but those go with a minus sign.

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Am I getting minus two lambda
equals zero?

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So.

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So I'm solving lambda cube plus
two lambda equals zero.

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So one root factors out lambda,
and the the rest is lambda

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squared plus two.

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OK.

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This is going the way we
expect, right?

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Because this gives the root
lambda equals zero,

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and gives the other two roots,
which are lambda equal what?

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The solutions of when is lambda
squared plus two equals zero

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then the eigenvalues those guys,
what are they?

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They're a multiple of i,
they're just square root of two

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i.

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When I set this equals to zero,
I have lambda squared equal to

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minus two, right?

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To make that zero?

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And the roots are square root
of two i and minus the square

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root of two i.

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So now I know what those are.

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I'll put those in,
now.

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Either the zero t is just a
one.

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That's just a one.

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This is square root of two I
and this is minus square root of

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two I.

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So, is the solution decaying to
zero?

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Is this a completely stable
problem where the solution is

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going to zero?

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No.

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In fact, all these things are
staying the same size.

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This thing is getting
multiplied by this number.

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e to the I something t,
that's a number that has

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magnitude one,
and sort of wanders around the

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unit circle.

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Same for this.

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So that the solution doesn't
blow up, and it doesn't go to

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zero.

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OK.

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And to find out what it
actually is, we would have to

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plug in initial conditions.

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But actually,
the next question I ask is,

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when does the solution return
to its initial value?

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I won't even say what's the
initial value.

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This is a case in which I think
this solution is periodic after.

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At t equals zero,
it starts with c1,

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c2, and c3, and then at some
value of t, it comes back to

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that.

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So that's a very special
question, Well,

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let's just take three seconds,
because that special question

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isn't likely to be on the quiz.

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But it comes back to the start,
when?

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Well, whenever we have e to the
two pi i, that's one,

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and we've come back again.

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So it comes back to the start.

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It's periodic,
when this square root of two i

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-- shall I call it capital T,
for the period?

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For that particular T,
if that equals two pi i,

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then e to this thing is one,
and we've come around again.

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So the period is T is
determined here,

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cancel the i-s,
and T is pi times the square

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root of two.

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So that's pretty neat.

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We get all the information
about all solutions,

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we haven't fixed on only one
particular solution,

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but it comes around again.

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So this was probably my first
chance to say something about

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the whole family of
anti-symmetric,

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skew-symmetric matrices.

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OK.

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And then, finally,
I asked, take two eigenvectors

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(again, I haven't computed the
eigenvectors) and it turns out

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they're orthogonal.

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They're orthogonal.

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The eigenvectors of a symmetric
matrix, or a skew-symmetric

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matrix, are always orthogonal.

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I guess may conscience makes me
tell you, what are all the

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matrices that have orthogonal
eigenvectors?

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And symmetric is the most
important class,

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so that's the one we've spoken
about.

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But let me just put that little
fact down, here.

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Orthogonal x-s.
eigenvectors.

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A matrix has orthogonal
eigenvectors,

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the exact condition -- it's
quite beautiful that I can tell

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you exactly when that happens.

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It happens when A times A
transpose equals A transpose

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times A.

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Any time that's the condition
for orthogonal eigenvectors.

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And because we're interested in
special families of vectors,

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tell me some special families
that fit.

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This is the whole requirement.

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That's a pretty special
requirement most matrices have.

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So the average three-by-three
matrix has three eigenvectors,

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but not orthogonal.

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But if it happens to commute
with its transpose,

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then, wonderfully,
the eigenvectors are

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orthogonal.

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Now, do you see how symmetric
matrices pass this test?

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Of course.

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If A transpose equals A,
then both sides are A squared,

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we've got it.

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How do anti-symmetric matrices
pass this test?

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If A transpose equals minus A,
then we've got it again,

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because we've got minus A
squared on both sides.

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So that's another group.

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And finally,
let me ask you about our other

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favorite family,
orthogonal matrices.

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Do orthogonal matrices pass
this test, if A is a Q,

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do they pass the test for
orthogonal eigenvectors.

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Well, if A is Q,
an orthogonal matrix,

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what is Q transpose Q?

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It's I.

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And what is Q Q transpose?

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It's I, we're talking square
matrices here.

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So yes, it passes the test.

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So the special cases are
symmetric, anti-symmetric (I'll

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say skew-symmetric,) and
orthogonal.

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Those are the three important
special classes that are in this

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family.

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OK.

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That's like a comment that,
could have been made back in,

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section six point four.

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OK, I can pursue the
differential equations,

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also this question,
didn't ask you to tell me,

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how would I find this matrix
exponential, e to the At?

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So can I erase this?

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I'll just stay with this
same...

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how would I find e to the At?

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Because, how does that come in?

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That's the key matrix for a
differential equation,

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because the solution is -- the
solution is u(t) is e^(At) u(0).

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So this is like the fundamental
matrix that multiplies the given

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function and gives the answer.

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00:16:44 --> 00:16:47
And how would we compute it if
we wanted that?

285
00:16:47 --> 00:16:52
We don't always have to find e
to the At, because I can go

286
00:16:52 --> 00:16:56
directly to the answer without
any e to the At-s,

287
00:16:56 --> 00:17:02
but hiding here is an e to the
At, and how would I compute it?

288
00:17:02 --> 00:17:05.66
Well, if A is diagonalizable.

289
00:17:05.66 --> 00:17:11
So I'm now going to put in my
usual if A can be diagonalized

290
00:17:11 --> 00:17:16
(and everybody remember that
there is an if there,

291
00:17:16 --> 00:17:21
because it might not have
enough eigenvectors) this

292
00:17:21 --> 00:17:27
example does have enough,
random matrices have enough.

293
00:17:27 --> 00:17:33
So if we can diagonalize,
then we get a nice formula for

294
00:17:33 --> 00:17:38
this, because an S comes way out
at the beginning,

295
00:17:38 --> 00:17:44
and S inverse comes way out at
the end, and we only have to

296
00:17:44 --> 00:17:48
take the exponential of lambda.

297
00:17:48 --> 00:17:52
And that's just a diagonal
matrix, so that's just e the

298
00:17:52 --> 00:17:55
lambda one t,
these guys are showing up,

299
00:17:55 --> 00:17:58
now, in e to the lambda nt.

300
00:17:58 --> 00:17:58.27
OK?

301
00:17:58.27 --> 00:18:01
That's a really quick review of
that formula.

302
00:18:01 --> 00:18:06
It's something we can compute
it quickly if we have done the S

303
00:18:06 --> 00:18:07
and lambda part.

304
00:18:07 --> 00:18:12
If we know S and lambda,
then it's not hard to take that

305
00:18:12 --> 00:18:13
step.

306
00:18:13 --> 00:18:17
OK, that's some comments on
differential equations.

307
00:18:17 --> 00:18:21
I would like to go on to a next
question that I started here.

308
00:18:21 --> 00:18:25
And it's, got several parts,
and I can just read it out.

309
00:18:25 --> 00:18:28
What we're given is a
three-by-three matrix,

310
00:18:28 --> 00:18:32
and we're told its eigenvalues,
except one of these is,

311
00:18:32 --> 00:18:35
like, we don't know,
and we're told the

312
00:18:35 --> 00:18:37
eigenvectors.

313
00:18:37 --> 00:18:41
And I want to ask you about the
matrix.

314
00:18:41 --> 00:18:41
OK.

315
00:18:41 --> 00:18:43
So, first question.

316
00:18:43 --> 00:18:46
Is the matrix diagonalizable?

317
00:18:46 --> 00:18:51.19
And I really mean for which c,
because I don't know c,

318
00:18:51.19 --> 00:18:56
so my questions will all be,
for which is there a condition

319
00:18:56 --> 00:18:59
on c, does one c work.

320
00:18:59 --> 00:19:06
But your answer should tell me
all the c-s that work.

321
00:19:06 --> 00:19:13
I'm not asking for you to tell
me, well, c equal four,

322
00:19:13 --> 00:19:16
yes, that checks out.

323
00:19:16 --> 00:19:23
I want to know all the c-s that
make it diagonalizable.

324
00:19:23 --> 00:19:24
OK?

325
00:19:24 --> 00:19:28
What's the real on
diagonalizable?

326
00:19:28 --> 00:19:32
We need enough eigenvectors,
right?

327
00:19:32 --> 00:19:38
We don't care what those
eigenvalues are,

328
00:19:38 --> 00:19:43
it's eigenvectors that count
for

329
00:19:43 --> 00:19:47.1
diagonalizable,
and we need three independent

330
00:19:47.1 --> 00:19:50
ones, and are those three guys
independent?

331
00:19:50 --> 00:19:50
Yes.

332
00:19:50 --> 00:19:53
Actually, let's look at them
for a moment.

333
00:19:53 --> 00:19:57
What do you see about those
three vectors right away?

334
00:19:57 --> 00:19:59
They're more than independent.

335
00:19:59 --> 00:20:03
Can you see why those three got
chosen?

336
00:20:03 --> 00:20:08
Because it will come up in the
next part, they're orthogonal.

337
00:20:08 --> 00:20:11
Those eigenvectors are
orthogonal.

338
00:20:11 --> 00:20:13
They're certainly independent.

339
00:20:13 --> 00:20:17
So the answer to diagonalizable
is, yes, all c,

340
00:20:17 --> 00:20:18
all c.

341
00:20:18 --> 00:20:21
Doesn't matter.
c could be a repeated guy,

342
00:20:21 --> 00:20:24
but we've got enough
eigenvectors,

343
00:20:24 --> 00:20:28
so that's what we care about.

344
00:20:28 --> 00:20:30
OK, second question.

345
00:20:30 --> 00:20:34.65
For which values of c is it
symmetric?

346
00:20:34.65 --> 00:20:38.43
OK, what's the answer to that
one?

347
00:20:38.43 --> 00:20:44
If we know the same setup if we
know that much about it,

348
00:20:44 --> 00:20:50
we know those eigenvectors,
and we've noticed they're

349
00:20:50 --> 00:20:56
orthogonal, then which c-s will
work?

350
00:20:56 --> 00:21:02
So the eigenvalues of that
symmetric matrix have to be

351
00:21:02 --> 00:21:02
real.

352
00:21:02 --> 00:21:04
So all real c.

353
00:21:04 --> 00:21:10
If c was i, the matrix wouldn't
have been symmetric.

354
00:21:10 --> 00:21:15
But if c is a real number,
then we've got real

355
00:21:15 --> 00:21:21
eigenvalues, we've got
orthogonal eigenvectors,

356
00:21:21 --> 00:21:25.59
that matrix is symmetric.

357
00:21:25.59 --> 00:21:27
OK, positive definite.

358
00:21:27 --> 00:21:31
OK, now this is a sub-case of
symmetric, so we need c to be

359
00:21:31 --> 00:21:35
real, so we've got a symmetric
matrix, but we also want the

360
00:21:35 --> 00:21:37
thing to be positive definite.

361
00:21:37 --> 00:21:41.31
Now, we're looking at
eigenvalues, we've got a lot of

362
00:21:41.31 --> 00:21:44
tests for positive definite,
but eigenvalues,

363
00:21:44 --> 00:21:47
if we know them,
is certainly a good,

364
00:21:47 --> 00:21:49.24
quick, clean test.

365
00:21:49.24 --> 00:21:52
Could this matrix be positive
definite?

366
00:21:52 --> 00:21:53
No.

367
00:21:53 --> 00:21:56
No, because it's got an
eigenvalue zero.

368
00:21:56 --> 00:21:59
It could be positive
semi-definite,

369
00:21:59 --> 00:22:05
you know, like consolation
prize, if c was greater or equal

370
00:22:05 --> 00:22:10
to zero, it would be positive
semi-definite.

371
00:22:10 --> 00:22:12
But it's not,
no.

372
00:22:12 --> 00:22:18.79
Semi-definite,
if I put that comment in,

373
00:22:18.79 --> 00:22:25
semi-definite,
that the condition would be c

374
00:22:25 --> 00:22:29
greater or equal to zero.

375
00:22:29 --> 00:22:32
That would be all right.

376
00:22:32 --> 00:22:33
OK.

377
00:22:33 --> 00:22:34
Next part.

378
00:22:34 --> 00:22:38
Is it a Markov matrix?

379
00:22:38 --> 00:22:40
Hm.

380
00:22:40 --> 00:22:46
Could this matrix be,
if I choose the number c

381
00:22:46 --> 00:22:49
correctly, a Markov matrix?

382
00:22:49 --> 00:22:55
Well, what do we know about
Markov matrices?

383
00:22:55 --> 00:23:01
Mainly, we know something about
their eigenvalues.

384
00:23:01 --> 00:23:08
One eigenvalue is always one,
and the other eigenvalues are

385
00:23:08 --> 00:23:09
smaller.

386
00:23:09 --> 00:23:12
Not larger.

387
00:23:12 --> 00:23:15
So an eigenvalue two can't
happen.

388
00:23:15 --> 00:23:19
So the answer is,
no, not a ma- that's never a

389
00:23:19 --> 00:23:20
Markov matrix.

390
00:23:20 --> 00:23:20.73
OK?

391
00:23:20.73 --> 00:23:23
And finally,
could one half of A be a

392
00:23:23 --> 00:23:25.36
projection matrix?

393
00:23:25.36 --> 00:23:29
So could it- could this --
eh-eh could this be twice a

394
00:23:29 --> 00:23:31
projection matrix?

395
00:23:31 --> 00:23:33
So let me write it this way.

396
00:23:33 --> 00:23:38
Could A over two be a
projection matrix?

397
00:23:38 --> 00:23:41
OK, what are projection
matrices?

398
00:23:41 --> 00:23:42
They're real.

399
00:23:42 --> 00:23:48
I mean, th- they're symmetric,
so their eigenvalues are real.

400
00:23:48 --> 00:23:52
But more than that,
we know what those eigenvalues

401
00:23:52 --> 00:23:53
have to be.

402
00:23:53 --> 00:24:00
What do the eigenvalues of a
projection matrix have to be?

403
00:24:00 --> 00:24:03
See, that any nice matrix we've
got an idea about its

404
00:24:03 --> 00:24:04
eigenvalues.

405
00:24:04 --> 00:24:07
So the eigenvalues of
projection matrices are zero and

406
00:24:07 --> 00:24:07
one.

407
00:24:07 --> 00:24:09
Zero and one,
only.

408
00:24:09 --> 00:24:12
Because P squared equals P,
let me call this matrix P,

409
00:24:12 --> 00:24:15
so P squared equals P,
so lambda squared equals

410
00:24:15 --> 00:24:19
lambda, because eigenvalues of P
squared are lambda squared,

411
00:24:19 --> 00:24:23
and we must have that,
so lambda equals zero or one.

412
00:24:23 --> 00:24:23.88
OK.

413
00:24:23.88 --> 00:24:30
Now what value of c will work
there?

414
00:24:30 --> 00:24:39
So, then, there are some value
that will work,

415
00:24:39 --> 00:24:47
and what will work?
c equals zero will work,

416
00:24:47 --> 00:24:54
or what else will work?
c equal to two.

417
00:24:54 --> 00:24:58
Because if c is two,
then when we divide by two,

418
00:24:58 --> 00:25:02
this Eigenvalue of two will
drop to one, and so will the

419
00:25:02 --> 00:25:04
other one, so,
or c equal to two.

420
00:25:04 --> 00:25:08
OK, those are the guys that
will work, and it was the fact

421
00:25:08 --> 00:25:12
that those eigenvectors were
orthogonal, the fact that those

422
00:25:12 --> 00:25:16.63
eigenvectors were orthogonal
carried us a lot of the way,

423
00:25:16.63 --> 00:25:17
here.

424
00:25:17 --> 00:25:22
If they weren't orthogonal,
then symmetric would have been

425
00:25:22 --> 00:25:26
dead, positive definite would
have been dead,

426
00:25:26 --> 00:25:29
projection would have been
dead.

427
00:25:29 --> 00:25:34.08
But those eigenvectors were
orthogonal, so it came down to

428
00:25:34.08 --> 00:25:36
the eigenvalues.

429
00:25:36 --> 00:25:44
OK, that was like a chance to
review a lot of this chapter.

430
00:25:44 --> 00:25:51.87
Shall I jump to the singular
value decomposition,

431
00:25:51.87 --> 00:25:58
then, as the third,
topic for, for the review?

432
00:25:58 --> 00:26:04
OK, so I'm going to.
jump to this.

433
00:26:04 --> 00:26:04
OK.

434
00:26:04 --> 00:26:10
So this is the singular value
decomposition,

435
00:26:10 --> 00:26:14
known to everybody as the SVD.

436
00:26:14 --> 00:26:22
And that's a factorization of A
into orthogonal times diagonal

437
00:26:22 --> 00:26:24
times orthogonal.

438
00:26:24 --> 00:26:31
And we always call those U and
sigma and V transpose.

439
00:26:31 --> 00:26:32
OK.

440
00:26:32 --> 00:26:38
And the key to that -- this is
for every matrix,

441
00:26:38 --> 00:26:42
every A, every A.

442
00:26:42 --> 00:26:45
Rectangular,
doesn't matter,

443
00:26:45 --> 00:26:49
whatever, has this
decomposition.

444
00:26:49 --> 00:26:52
So it's really important.

445
00:26:52 --> 00:26:58
And the key to it is to look at
things like A transpose A.

446
00:26:58 --> 00:27:05
Can we remember what happens
with A transpose A?

447
00:27:05 --> 00:27:12
If I just transpose that I get
V sigma transpose U transpose,

448
00:27:12 --> 00:27:17
that's multiplying A,
which is U, sigma V transpose,

449
00:27:17 --> 00:27:24
and the result is V on the
outside, s- U transpose U is the

450
00:27:24 --> 00:27:30
identity, because it's an
orthogonal matrix.

451
00:27:30 --> 00:27:35
So I'm just left with sigma
transpose sigma in the middle,

452
00:27:35 --> 00:27:39
that's a diagonal,
possibly rectangular diagonal

453
00:27:39 --> 00:27:42
by its transpose,
so the result,

454
00:27:42 --> 00:27:46
this is orthogonal,
diagonal, orthogonal.

455
00:27:46 --> 00:27:50
So, I guess,
actually, this is the SVD for A

456
00:27:50 --> 00:27:51
transpose A.

457
00:27:51 --> 00:27:57
Here I see orthogonal,
diagonal, and orthogonal.

458
00:27:57 --> 00:27:57
Great.

459
00:27:57 --> 00:28:01.54
But a little more is happening.

460
00:28:01.54 --> 00:28:05
For A transpose A,
the difference is,

461
00:28:05 --> 00:28:09
the orthogonal guys are the
same.

462
00:28:09 --> 00:28:12
It's V and V transpose.

463
00:28:12 --> 00:28:14
What I seeing here?

464
00:28:14 --> 00:28:20
I'm seeing the factorization
for a symmetric matrix.

465
00:28:20 --> 00:28:25
This thing is symmetric.

466
00:28:25 --> 00:28:28
So in a symmetric case,
U is the same as V.

467
00:28:28 --> 00:28:31
U is the same as V for this
symmetric matrix,

468
00:28:31 --> 00:28:34
and, of course,
we see it happening.

469
00:28:34 --> 00:28:34
OK.

470
00:28:34 --> 00:28:37
So that tells us,
right away, what V is.

471
00:28:37 --> 00:28:40
V is the eigenvector matrix for
A transpose A.

472
00:28:40 --> 00:28:40
OK.

473
00:28:40 --> 00:28:44
Now, if you were here when I
lectured about this topic,

474
00:28:44 --> 00:28:48
when I gave the topic on
singular value decompositions,

475
00:28:48 --> 00:28:53.22
you'll remember that I got into
trouble.

476
00:28:53.22 --> 00:29:00.65
I'm sorry to remember that
myself, but it happened.

477
00:29:00.65 --> 00:29:01
OK.

478
00:29:01 --> 00:29:03
How did it happen?

479
00:29:03 --> 00:29:10
I was in great shape for a
while, cruising along.

480
00:29:10 --> 00:29:17
So I found the eigenvectors for
A transpose A.

481
00:29:17 --> 00:29:19
Good.

482
00:29:19 --> 00:29:23
I found the singular values,
what were they?

483
00:29:23 --> 00:29:25
What were the singular values?

484
00:29:25 --> 00:29:29
The singular value number i,
or -- these are the guys in

485
00:29:29 --> 00:29:33
sigma -- this is diagonal with
the number sigma in it.

486
00:29:33 --> 00:29:37
This diagonal is sigma one,
sigma two, up to the rank,

487
00:29:37 --> 00:29:41
sigma r, those are the non-zero
ones.

488
00:29:41 --> 00:29:45
So I found those,
and what are they?

489
00:29:45 --> 00:29:47
Remind me about that?

490
00:29:47 --> 00:29:52.9
Well, here, I'm seeing them
squared, so their squares are

491
00:29:52.9 --> 00:29:56
the eigenvalues of A transpose
A.

492
00:29:56 --> 00:29:56
Good.

493
00:29:56 --> 00:30:02
So I just take the square root,
if I want the eigenvalues of A

494
00:30:02 --> 00:30:08
transpose --
If I want the sigmas and I know

495
00:30:08 --> 00:30:13
these, I take the square root,
the positive square root.

496
00:30:13 --> 00:30:14
OK.

497
00:30:14 --> 00:30:17
Where did I run into trouble?

498
00:30:17 --> 00:30:20
Well, then, my final step was
to find U.

499
00:30:20 --> 00:30:23
And I didn't read the book.

500
00:30:23 --> 00:30:28
So, I did something that was
practically right,

501
00:30:28 --> 00:30:33
but --
well, I guess practically right

502
00:30:33 --> 00:30:35
is not quite the same.

503
00:30:35 --> 00:30:41.1
OK, so I thought,
OK, I'll look at A A transpose.

504
00:30:41.1 --> 00:30:45
What happened when I looked at
A A transpose?

505
00:30:45 --> 00:30:51
Let me just put it here,
and then I can feel it.

506
00:30:51 --> 00:30:55.14
OK, so here's A A transpose.

507
00:30:55.14 --> 00:31:00
So that's U sigma V transpose,
that's A, and then the

508
00:31:00 --> 00:31:06.49
transpose is V sigma transpose,
U sigma transpose.

509
00:31:06.49 --> 00:31:07
Fine.

510
00:31:07 --> 00:31:12.11
And then, in the middle is the
identity again,

511
00:31:12.11 --> 00:31:14
so it looks great.

512
00:31:14 --> 00:31:18
U sigma sigma transpose,
U transpose.

513
00:31:18 --> 00:31:19
Fine.

514
00:31:19 --> 00:31:25.05
All good, and now these columns
of U are the eigenvectors,

515
00:31:25.05 --> 00:31:29
that's U is the eigenvector
matrix for this guy.

516
00:31:29 --> 00:31:32
That was correct,
so I did that fine.

517
00:31:32 --> 00:31:35
Where did something go wrong?

518
00:31:35 --> 00:31:36.62
A sign went wrong.

519
00:31:36.62 --> 00:31:41
A sign went wrong because --
and now -- now I see,

520
00:31:41 --> 00:31:44
actually, somebody told me
right after class,

521
00:31:44 --> 00:31:48
we can't tell from this
description which sign to give

522
00:31:48 --> 00:31:50.23
the eigenvectors.

523
00:31:50.23 --> 00:31:53
If these are the eigenvectors
of this matrix,

524
00:31:53 --> 00:31:57
well, if you give me an
eigenvector and I change all its

525
00:31:57 --> 00:32:01
signs, we've still got another
eigenvector.

526
00:32:01 --> 00:32:05
So what I wasn't able to
determine (and I had a

527
00:32:05 --> 00:32:09
fifty-fifty change and life let
me down,) the signs I just

528
00:32:09 --> 00:32:11
happened to pick for the
eigenvectors,

529
00:32:11 --> 00:32:14
one of them I should have
reversed the sign.

530
00:32:14 --> 00:32:17
So, from this,
I can't tell whether the

531
00:32:17 --> 00:32:22
eigenvector or its negative is
the right one to use in there.

532
00:32:22 --> 00:32:28
So the right way to do it is
to, having settled on the signs,

533
00:32:28 --> 00:32:32.26
the Vs also,
I don't know which sign to

534
00:32:32.26 --> 00:32:34
choose, but I choose one.

535
00:32:34 --> 00:32:36
I choose one.

536
00:32:36 --> 00:32:41
And then, instead,
I should have used the one that

537
00:32:41 --> 00:32:47
tells me what sign to choose,
the rule that A times a V is

538
00:32:47 --> 00:32:50
sigma times the U.

539
00:32:50 --> 00:32:55
So, having decided on the V,
I multiply by A,

540
00:32:55 --> 00:33:02
I'll notice the factor sigma
coming out, and there will be a

541
00:33:02 --> 00:33:08
unit vector there,
and I now know exactly what it

542
00:33:08 --> 00:33:14
is, and not only up to a change
of sign.

543
00:33:14 --> 00:33:19
So that's the good and,
of course, this is the main

544
00:33:19 --> 00:33:21
point about the SVD.

545
00:33:21 --> 00:33:25
That's the point that we've
diagonalized,

546
00:33:25 --> 00:33:31
that's A times the matrix of Vs
equals U times the diagonal

547
00:33:31 --> 00:33:33
matrix of sigmas.

548
00:33:33 --> 00:33:36
That's the same as that.

549
00:33:36 --> 00:33:36
OK.

550
00:33:36 --> 00:33:40
So that's, like,
correcting the wrong sign from

551
00:33:40 --> 00:33:42
that earlier lecture.

552
00:33:42 --> 00:33:48
And that would complete that,
so that's how you would compute

553
00:33:48 --> 00:33:48
the SVD.

554
00:33:48 --> 00:33:52
Now, on the quiz,
I going to ask -- well,

555
00:33:52 --> 00:33:55
maybe on the final.

556
00:33:55 --> 00:33:58
So we've got quiz and final
ahead.

557
00:33:58 --> 00:34:03
Sometimes, you might be asked
to find the SVD if I give you

558
00:34:03 --> 00:34:08
the matrix -- let me come back,
now, to the main board -- or,

559
00:34:08 --> 00:34:11
I might give you the pieces.

560
00:34:11 --> 00:34:15
And I might ask you something
about the matrix.

561
00:34:15 --> 00:34:18
For example,
suppose I ask you,

562
00:34:18 --> 00:34:21
oh, let's say,
if I tell you what sigma is --

563
00:34:21 --> 00:34:23
OK.

564
00:34:23 --> 00:34:27
Let's take one example.

565
00:34:27 --> 00:34:36
Suppose sigma is -- so all
that's how we would compute

566
00:34:36 --> 00:34:37
them.

567
00:34:37 --> 00:34:43.26
But now, suppose I give you
these.

568
00:34:43.26 --> 00:34:52
Suppose I give you sigma is,
say, three two.

569
00:34:52 --> 00:34:55
And I tell you that U has a
couple of columns,

570
00:34:55 --> 00:34:57
and V has a couple of columns.

571
00:34:57 --> 00:34:58
OK.

572
00:34:58 --> 00:35:02
Those are orthogonal columns,
of course, because U and V are

573
00:35:02 --> 00:35:03
orthogonal.

574
00:35:03 --> 00:35:07
I'm just sort of,
like, getting you to think

575
00:35:07 --> 00:35:10
about the SVD,
because we only had that one

576
00:35:10 --> 00:35:12
lecture about it,
and one homework,

577
00:35:12 --> 00:35:17.44
and, what kind of a matrix have
I got here?

578
00:35:17.44 --> 00:35:21
What do I know about this
matrix?

579
00:35:21 --> 00:35:27
All I really know right now is
that its singular values,

580
00:35:27 --> 00:35:34.01
those sigmas are three and two,
and the only thing interesting

581
00:35:34.01 --> 00:35:40
that I can see in that is that
they're not zero.

582
00:35:40 --> 00:35:43
I know that this matrix is
non-singular,

583
00:35:43 --> 00:35:44
right?

584
00:35:44 --> 00:35:47
That's invertible,
I don't have any zero

585
00:35:47 --> 00:35:52
eigenvalues, and zero singular
values, that's invertible,

586
00:35:52 --> 00:35:57
there's a typical SVD for a
nice two-by-two non-singular

587
00:35:57 --> 00:36:00
invertible good matrix.

588
00:36:00 --> 00:36:05
If I actually gave you a
matrix, then you'd have to find

589
00:36:05 --> 00:36:08
the Us and the Vs as we just
spoke.

590
00:36:08 --> 00:36:09
But, there.

591
00:36:09 --> 00:36:13
Now, what if the two wasn't a
two but it was -- well,

592
00:36:13 --> 00:36:18
let me make an extreme case,
here -- suppose it was minus

593
00:36:18 --> 00:36:19
five.

594
00:36:19 --> 00:36:21.43
That's wrong,
right away.

595
00:36:21.43 --> 00:36:24
That's not a singular value
decomposition,

596
00:36:24 --> 00:36:25
right?

597
00:36:25 --> 00:36:27
The singular values are not
negative.

598
00:36:27 --> 00:36:31
So that's not a singular value
decomposition,

599
00:36:31 --> 00:36:32
and forget it.

600
00:36:32 --> 00:36:32
OK.

601
00:36:32 --> 00:36:35
So let me ask you about that
one.

602
00:36:35 --> 00:36:39
What can you tell me about that
matrix?

603
00:36:39 --> 00:36:41
It's singular,
right?

604
00:36:41 --> 00:36:47
It's got a singular matrix
there in the middle,

605
00:36:47 --> 00:36:51
and, let's see,
so, OK, it's singular,

606
00:36:51 --> 00:36:55
maybe you can tell me,
its rank?

607
00:36:55 --> 00:36:58
What's the rank of A?

608
00:36:58 --> 00:37:03.46
It's clearly -- somebody just
say it -- one,

609
00:37:03.46 --> 00:37:05
thanks.

610
00:37:05 --> 00:37:08
The rank is one,
so the null space,

611
00:37:08 --> 00:37:12
what's the dimension of the
null space?

612
00:37:12 --> 00:37:12.68
One.

613
00:37:12.68 --> 00:37:13
Right?

614
00:37:13 --> 00:37:18
We've got a two-by-two matrix
of rank one, so of all that

615
00:37:18 --> 00:37:23
stuff from the beginning of the
course is still with us.

616
00:37:23 --> 00:37:27
The dimensions of those
fundamental spaces is still

617
00:37:27 --> 00:37:31
central, and a basis for them.

618
00:37:31 --> 00:37:37
Now, can you tell me a vector
that's in the null space?

619
00:37:37 --> 00:37:43
And then that will be my last
point to make about the SVD.

620
00:37:43 --> 00:37:49
Can you tell me a vector that's
in the null space?

621
00:37:49 --> 00:37:55.15
So what would I multiply by and
get zero, here?

622
00:37:55.15 --> 00:37:58
I think the answer is probably
v2.

623
00:37:58 --> 00:38:03
I think probably v2 is in the
null space, because I think that

624
00:38:03 --> 00:38:08
must be the eigenvector going
with this zero eigenvalue.

625
00:38:08 --> 00:38:09
Yes.

626
00:38:09 --> 00:38:10
Have a look at that.

627
00:38:10 --> 00:38:16
And I could ask you the null
space of A transpose.

628
00:38:16 --> 00:38:19
And I could ask you the column
space.

629
00:38:19 --> 00:38:21
All that stuff.

630
00:38:21 --> 00:38:25.26
Everything is sitting there in
the SVD.

631
00:38:25.26 --> 00:38:29
The SVD takes a little more
time to compute,

632
00:38:29 --> 00:38:34
but it displays all the good
stuff about a matrix.

633
00:38:34 --> 00:38:34
OK.

634
00:38:34 --> 00:38:38
Any question about the SVD?

635
00:38:38 --> 00:38:44
Let me keep going with further
topics.

636
00:38:44 --> 00:38:47
Now, let's see.

637
00:38:47 --> 00:38:56
Similar matrices we've talked
about, let me see if I've got

638
00:38:56 --> 00:38:58.91
another, -- OK.

639
00:38:58.91 --> 00:39:05
Here's a true false,
so we can do that,

640
00:39:05 --> 00:39:06
easily.

641
00:39:06 --> 00:39:06
So.

642
00:39:06 --> 00:39:11
Question, A given.

643
00:39:11 --> 00:39:14
A is symmetric and orthogonal.

644
00:39:14 --> 00:39:15
OK.

645
00:39:15 --> 00:39:21
So beautiful matrices like that
don't come along every day.

646
00:39:21 --> 00:39:26
But what can we say first about
its eigenvalues?

647
00:39:26 --> 00:39:28
Actually, of course.

648
00:39:28 --> 00:39:34
Here are our two most important
classes of matrices,

649
00:39:34 --> 00:39:39
and we're looking at the
intersection.

650
00:39:39 --> 00:39:44
So those really are neat
matrices, and what can you tell

651
00:39:44 --> 00:39:48
me about what could the possible
eigenvalues be?

652
00:39:48 --> 00:39:51
Eigenvalues can be what?

653
00:39:51 --> 00:39:55.55
What do I know about the
eigenvalues of a symmetric

654
00:39:55.55 --> 00:39:56.17
matrix?

655
00:39:56.17 --> 00:39:57
Lambda is real.

656
00:39:57 --> 00:40:02
What do I know about the
eigenvalues of an orthogonal

657
00:40:02 --> 00:40:03
matrix?

658
00:40:03 --> 00:40:04
Ha.

659
00:40:04 --> 00:40:05
Maybe nothing.

660
00:40:05 --> 00:40:08.23
But, no, that can't be.

661
00:40:08.23 --> 00:40:14
What do I know about the
eigenvalues of an orthogonal

662
00:40:14 --> 00:40:14
matrix?

663
00:40:14 --> 00:40:17
Well, what feels right?

664
00:40:17 --> 00:40:23
Basing mathematics on just a
little gut instinct here,

665
00:40:23 --> 00:40:29
the eigenvalues of an
orthogonal matrix ought to have

666
00:40:29 --> 00:40:31
magnitude one.

667
00:40:31 --> 00:40:38
Orthogonal matrices are like
rotations, they're not changing

668
00:40:38 --> 00:40:44
the length, so orthogonal,
the eigenvalues are one.

669
00:40:44 --> 00:40:47.16
Let me just show you why.

670
00:40:47.16 --> 00:40:47
Why?

671
00:40:47 --> 00:40:52
So the matrix,
can I call it Q for orthogonal

672
00:40:52 --> 00:40:55
for the moment?

673
00:40:55 --> 00:40:59
If I look at Q x equal lambda
x, how do I see that this thing

674
00:40:59 --> 00:41:01
has magnitude one?

675
00:41:01 --> 00:41:03
I take the length of both
sides.

676
00:41:03 --> 00:41:06
This is taking lengths,
taking lengths,

677
00:41:06 --> 00:41:10
this is whatever the magnitude
is times the length of x.

678
00:41:10 --> 00:41:14
And what's the length of Q x if
Q is an orthogonal matrix?

679
00:41:14 --> 00:41:16
This is something you should
know.

680
00:41:16 --> 00:41:20
It's the same as the length of
x.

681
00:41:20 --> 00:41:24
Orthogonal matrices don't
change lengths.

682
00:41:24 --> 00:41:26
So lambda has to be one.

683
00:41:26 --> 00:41:27
Right.

684
00:41:27 --> 00:41:27
OK.

685
00:41:27 --> 00:41:33
That's worth committing to
memory, that could show up

686
00:41:33 --> 00:41:33.99
again.

687
00:41:33.99 --> 00:41:34
OK.

688
00:41:34 --> 00:41:38
So what's the answer now to
this question,

689
00:41:38 --> 00:41:42
what can the eigenvalues be?

690
00:41:42 --> 00:41:48
There's only two possibilities,
and they are one and the other

691
00:41:48 --> 00:41:52
one, the other possibility is
negative one,

692
00:41:52 --> 00:41:56
right, because these have the
right magnitude,

693
00:41:56 --> 00:41:58
and they're real.

694
00:41:58 --> 00:41:58
OK.

695
00:41:58 --> 00:41:59
TK.
true -- OK.

696
00:41:59 --> 00:42:01
True or false?

697
00:42:01 --> 00:42:04
A is sure to be positive
definite.

698
00:42:04 --> 00:42:08
Well, this is a great matrix,
but

699
00:42:08 --> 00:42:11
is it sure to be positive
definite?

700
00:42:11 --> 00:42:11
No.

701
00:42:11 --> 00:42:16.12
If it could have an eigenvalue
minus one, it wouldn't be

702
00:42:16.12 --> 00:42:17.58
positive definite.

703
00:42:17.58 --> 00:42:21
True or false,
it has no repeated eigenvalues.

704
00:42:21 --> 00:42:22
That's false,
too.

705
00:42:22 --> 00:42:27
In fact, it's going to have
repeated eigenvalues if it's as

706
00:42:27 --> 00:42:29.84
big
as three by three,

707
00:42:29.84 --> 00:42:33.6
one of these c- one of these,
at least, will have to get

708
00:42:33.6 --> 00:42:34
repeated.

709
00:42:34 --> 00:42:34
Sure.

710
00:42:34 --> 00:42:37
So it's got repeated
eigenvalues, but,

711
00:42:37 --> 00:42:38
is it diagonalizable?

712
00:42:38 --> 00:42:41
It's got these many,
many, repeated eigenvalues.

713
00:42:41 --> 00:42:45
If it's fifty by fifty,
it's certainly got a lot of

714
00:42:45 --> 00:42:46
repetitions.

715
00:42:46 --> 00:42:48
Is it diagonalizable?

716
00:42:48 --> 00:42:48
Yes.

717
00:42:48 --> 00:42:53
All symmetric matrices,
all orthogonal matrices can be

718
00:42:53 --> 00:42:54
diagonalized.

719
00:42:54 --> 00:42:57
And, in fact,
the eigenvectors can even be

720
00:42:57 --> 00:42:58.67
chosen orthogonal.

721
00:42:58.67 --> 00:43:00
So it could be,
sort of, like,

722
00:43:00 --> 00:43:06
diagonalized the best way with
a Q, and not just any old S.

723
00:43:06 --> 00:43:06
OK.

724
00:43:06 --> 00:43:09
Is it non-singular?

725
00:43:09 --> 00:43:15.18
Is a symmetric orthogonal
matrix non-singular?

726
00:43:15.18 --> 00:43:15
Sure.

727
00:43:15 --> 00:43:21
Orthogonal matrices are always
non-singular.

728
00:43:21 --> 00:43:26.24
And, obviously,
we don't have any zero

729
00:43:26.24 --> 00:43:27
Eigenvalues.

730
00:43:27 --> 00:43:31
Is it sure to be
diagonalizable?

731
00:43:31 --> 00:43:34
Yes.

732
00:43:34 --> 00:43:42
Now, here's a final step --
show that one-half of A plus I

733
00:43:42 --> 00:43:48
is A -- that is,
prove one-half of A plus I is a

734
00:43:48 --> 00:43:51
projection matrix.

735
00:43:51 --> 00:43:51
OK?

736
00:43:51 --> 00:43:53.41
Let's see.

737
00:43:53.41 --> 00:43:55
What do I do?

738
00:43:55 --> 00:43:59
I could see two ways to do
this.

739
00:43:59 --> 00:44:07
I could check the properties of
a projection matrix,

740
00:44:07 --> 00:44:11
which are what?

741
00:44:11 --> 00:44:13
A projection matrix is
symmetric.

742
00:44:13 --> 00:44:18
Well, that's certainly
symmetric, because A is.

743
00:44:18 --> 00:44:20
And what's the other property?

744
00:44:20 --> 00:44:24
I should square it,
and hopefully get the same

745
00:44:24 --> 00:44:25
thing back.

746
00:44:25 --> 00:44:29
So can I do that,
square and see if I get the

747
00:44:29 --> 00:44:32
same thing back?

748
00:44:32 --> 00:44:37
So if I square it,
I'll get one-quarter of A

749
00:44:37 --> 00:44:41
squared plus two A plus I,
right?

750
00:44:41 --> 00:44:47.43
And the question is,
does that agree with p- the

751
00:44:47.43 --> 00:44:49
thing itself?

752
00:44:49 --> 00:44:51
One-half A plus I.

753
00:44:51 --> 00:44:51
Hm.

754
00:44:51 --> 00:44:57
I guess I'd like to know
something about A squared.

755
00:44:57 --> 00:45:00
What is A squared?

756
00:45:00 --> 00:45:04
That's our problem.

757
00:45:04 --> 00:45:05
What is A squared?

758
00:45:05 --> 00:45:11
If A is symmetric and
orthogonal, A is symmetric and

759
00:45:11 --> 00:45:12
orthogonal.

760
00:45:12 --> 00:45:15
This is what we're given,
right?

761
00:45:15 --> 00:45:19
It's symmetric,
and it's orthogonal.

762
00:45:19 --> 00:45:21
So what's A squared?

763
00:45:21 --> 00:45:21
I.

764
00:45:21 --> 00:45:26.13
A squared is I,
because A times A -- if A

765
00:45:26.13 --> 00:45:31
equals its own inverse,
so A times A is the same as A

766
00:45:31 --> 00:45:36
times A inverse,
which is I.

767
00:45:36 --> 00:45:39
So this A squared here is I.

768
00:45:39 --> 00:45:42
And now we've got it.

769
00:45:42 --> 00:45:48
We've got two identities over
four, that's good,

770
00:45:48 --> 00:45:54
and we've got two As over four,
that's good.

771
00:45:54 --> 00:45:54
OK.

772
00:45:54 --> 00:46:02
So it turned out to be a
projection matrix safely.

773
00:46:02 --> 00:46:07
And we could also have said,
well, what are the eigenvalues

774
00:46:07 --> 00:46:08
of this thing?

775
00:46:08 --> 00:46:12
What are the eigenvalues of a
half A plus I?

776
00:46:12 --> 00:46:16
If the eigenvalues of A are one
and minus one,

777
00:46:16 --> 00:46:19
what are the eigenvalues of A
plus I?

778
00:46:19 --> 00:46:24
Just stay with it these last
thirty seconds here.

779
00:46:24 --> 00:46:28
What if I know these
eigenvalues of A,

780
00:46:28 --> 00:46:32
and I add the identity,
the eigenvalues of A plus I are

781
00:46:32 --> 00:46:34
zero and two.

782
00:46:34 --> 00:46:39
And then when I divide by two,
the eigenvalues are zero and

783
00:46:39 --> 00:46:39
one.

784
00:46:39 --> 00:46:43
So it's symmetric,
it's got the right eigenvalues,

785
00:46:43 --> 00:46:47
it's a projection matrix.

786
00:46:47 --> 00:46:51
OK, you're seeing a lot of
stuff about eigenvalues,

787
00:46:51 --> 00:46:55
and special matrices,
and that's what the quiz is

788
00:46:55 --> 00:46:56
about.

789
00:46:56 --> 00:46:59
OK, so good luck on the quiz.