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Whoop de doo!
Let’s try something more interesting. Suppose we put 1 in box B3 and in B4
put '=B2+B3'. Now copy B4 into B5…B1000
What will appear are what are called the Fibonacci numbers.
So?
What are their properties? They have lots and lots of them. We can observe
that they grow without limit. But suppose we look at the ratio of two successive
Fibonacci numbers. To do this put in C5: 'B5/b4'(case doesn’t matter) and copy
this down column C. What do you see?
What is that number?
The Fibonacci numbers and the positive integers as well each form an infinite
sequence; in column C you see what is called a convergent sequence.
To figure out what the number you see is, imagine that we had exactly f(n) = rf(n
- 1) for some r. What can we say about r? Since the defining property of the Fibonacci
numbers is f(n) = f(n - 1) + f(n - 2), we could then
write r2 f(n - 2) = r f(n - 2) + f(n - 2) , which we can solve for
r.
The larger solution is called the "Golden Ratio".
By the way, the Fibonacci numbers count the number of different ways of inserting
n dominoes into a 2 by n grid, so that each domino covers two adjacent boxes.
Exercises:
0.1 Set this all up on your own machine. Solution
0.2 Prove that the Fibonacci numbers count the number of different ways
of inserting n dominoes into a 2 by n grid, so that each domino covers two adjacent
boxes. Solution
0.3 Make a definition of convergence of a sequence that reflects the property
that you see. Solution
0.4 This procedure produces a solution to the quadratic equation indicated
above. Given any quadratic with integer coefficients, we can produce a recursion
as above and by substituting it into B4 and copying it down, look at what happens
to it. Try doing this with some quadratics, and find another for which we get
a solution, and one which we don’t. What happens with the cubic x3
= x + 1? Solution
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