WEBVTT
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PROFESSOR: Hey, we're back.
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Today we're going to do a
singular value decomposition
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question.
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The problem is really
simple to state:
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find the singular value
decomposition of this matrix
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C equals [5, 5; -1, 7].
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Hit pause, try it yourself,
I'll be back in a minute
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and we can do it together.
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All right, we're back,
now let's do it together.
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Now, I know Professor
Strang has done
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a couple of these in lecture,
but as he pointed out there,
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it's really easy
to make a mistake,
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so you can never do enough
examples of finding the SVD.
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So, what does the SVD look like?
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What do we want to end up with?
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Well, we want a decomposition
C equals U sigma V transpose.
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U and V are going to be
orthogonal matrices, that
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is, their columns
are orthonormal sets.
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Sigma is going to
be a diagonal matrix
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with non-negative entries.
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OK, good.
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So now, how do we find
this decomposition?
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Well, we need two equations, OK?
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One is C transpose C is equal
to V, sigma transpose, sigma,
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V transpose.
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And you get this just by
plugging in C transpose C
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here and noticing that U
transpose U is 1, since U
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is an orthogonal matrix.
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Okay.
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And the second equation is just
noticing that V transpose is V
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inverse, and moving it to the
other side of the equation,
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which is C*V equals U*sigma.
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OK, so these are
the two equations
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we need to use to find
V, sigma, and U. OK,
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so let's start
with the first one.
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Let's compute C transpose
C. So C transpose C
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is that-- Well, if
you compute, we'll
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get a 26, an 18, an
18, and a 74, great.
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Now, what you notice
about this equation
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is this is just a
diagonalization of C transpose
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C. So we need to find
the eigenvalues-- those
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will be the entries
of sigma transpose
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sigma-- and the
eigenvectors which will be
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the columns of a V. Okay, good.
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So how do we find those?
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Well, we look at the determinant
of C transpose C minus lambda
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times the identity,
which is the determinant
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of 26 minus lambda, 18, 18,
and 74-- 74 minus lambda,
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thank you.
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Good, OK, and what
is that polynomial?
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Well, we get a lambda squared,
now the 26 plus 74 is 100,
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so minus 100*lambda.
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And I'll let you do 26 times 74
minus 18 squared on your own,
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but you'll see you get 1,600,
and this easily factors
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as lambda minus 20
times lambda minus 80.
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So the eigenvalues
are 20 and 80.
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Now what are the eigenvectors?
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Well, you take C transpose C
minus 20 times the identity,
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and you get 6, 18, 18 and 54.
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To find the eigenvector
with eigenvalue 20,
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we need to find a vector in
the null space of this matrix.
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Note that the second
column is three times
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the first column, so our
first vector, v_1, we can just
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take that to be, well, we
could take it to be [-3, 1],
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but we'd like it to be
a unit vector, right?
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Remember the columns of
v should be unit vectors
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because they're orthonormal.
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So 3 squared plus
1 squared is 10,
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we have to divide by
the square root of 10.
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OK, similarly, we do C transpose
C minus 80 times the identity,
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we'll get -54, 18; 18,
and -6, and similarly
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we can find that v_2 will
be 1 over square root of 10,
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3 over the square root of 10.
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Great, OK, so what
information do we have now?
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we have our V matrix, which
is just made up of these two
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columns, and we actually
have our sigma matrix too,
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because the squares of the
diagonal entries of sigma
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are 20 and 80.
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Good, so let's write those
down, write down what we have.
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So we have V-- I just
add these vectors
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and make them the
columns of my matrix.
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Square root of 10, 1
over square root of 10;
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1 over square root of 10,
3 over square root of 10.
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And sigma, this is just the
square roots of 20 and 80,
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which is just 2 root 5 and
4 root 5 along the diagonal.
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Squeezing it in here, I hope
you all can see these two.
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Good, so these are two of the
three parts of my singular
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value decomposition.
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The last thing I
need to find is u,
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and for that I need to use this
second equation right here.
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So you need to multiply
C times V, okay so So
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c is [5, 5; -1, 7],
let's multiply it by V,
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1 over root 10, 3 over
square root of 10.
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What do we get?
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Well, I'll let you
work out the details,
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but it's not hard here.
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You get -10 over root 10, which
is just negative square root
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of 10 here.
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Then I just get 2 square
root of 10, and then I get--
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1 is 2 square root of 10 and--
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I think I made an error here.
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Give me a second to look
through my computation again.
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AUDIENCE: [INAUDIBLE]
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PROFESSOR: The (2, 1) entry
should be-- oh, yes, thank you.
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The (2, 1) entry should
be the square root of 10.
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Good, yes, that's what I was
hoping, yes, because we get--
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Yes, I did it in
the wrong order,
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right, so your recitation
instructor should
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know how to multiply matrices,
great, yes, thank you.
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You multiply this first, then
this, then this, and then this,
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and if you do it correctly
you will get this matrix here.
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Good, great.
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So now I'd like this
to be my U matrix,
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but it's actually U times sigma,
so I need to make these entries
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unit length.
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OK, so I get -1 over root 2, 1
over root 2, 1 over root 2, 1
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over root 2, times
my sigma matrix
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here, which is, remember,
2 square root of 5,
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4 square root of 5,
and these constants
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are just what I needed to
divide these columns by in order
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to make them unit vectors.
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So now, here's my U matrix,
1 over square root of 2,
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-1 over square root of 2;
1 over square root of 2,
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1 over square root of 2, good.
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So now I have all three
matrices, U, V, and sigma
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and despite some little
errors here and there,
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I think this is actually right.
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You should go check it
yourself, because if you're
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at all like me, you've screwed
up several times by now.
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But anyway, this is
a good illustration
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of how to find the singular
value decomposition.
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Recall that you're
looking for U sigma V
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transpose where u and v
are orthogonal matrices,
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and sigma is diagonal
with non-negative entries.
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And you find it using
these two equations,
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you compute C transpose C,
that's V sigma transpose sigma
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times V transpose, and you
also have C*V is U*sigma.
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I hope this was a
helpful illustration.