# CSCE 221.509 | Fibonacci and Recursion

Natural numbers apparently don’t necessarily involve the number zero

Definition of Natural Numbers
Base case: n=0
n=0 is a natural number
Successive case: n_{i+1} = n_i + 1
if n_i is a natural number, then n_i + 1 is a natural number.


## Fibonacci sequence

There was the rabbit Fibonacci sequence analogy

F(0) = 1
F(1) = 1
F(2) = F(1) + F(0)
F(3) = F(2) + F(1)
F(4) = F(3) + F(2)
F(n) = F(n-1) + F(n-2), for n > 1

Algorithm Fib(n)
Input: n //rank of fib. number
Output: actual fib. number

if n < 2 then
return 1;
end if
return F(n-1) + Fib(n-2)


Fib(5) = (Fib(4) + Fib(3)) = ((Fib(3) + Fib(2)) + (Fib(2) + Fib(1)))

Do the analysis to speed it up.

T(n)
T(0) = 1
T(1) = 1
T(n-1) + T(n-2)
So T(n) = T(n-1) + T(n-2) + 1
T(n) = 2F(n) - 1


Note: T(n-2) < T(n-1), for n >= 3

T(n) < T(n-1) + T(n-1) +1

T(n) < 2T(n-1) + 1

Note: T(n-1) < 2T(n-2) + 1

T(n) < 2^2 T(n-2) + 2 + 1

Note: T(n-2) < 2T(n-3) + 1

T(n) < 2^3T(n-3) + 2^2 + 2 + 1

$$T(n) \lt 2^k(n-k) + (2^{k-1} + \dots + 2^2 + 2 + 1)$$

Side note: how to solve

$$S(k) = (2^{k-1} + \dots + 2^2 + 2 + 1)$$

$$2 \ast S(k) = (2^k + \dots + 2^3 + 2^2 + 2)$$

$$2 \ast S(k) - S(k) = 2^k + 2^{k-1} - 2^{k-1} + … + 2^2 - 2^2 + 2 - 2 + 1$$

So $$S(k) = 2^k + 1$$

Therefore

$$T(n) \lt 2^k \ast T(n-k) + (2^k + 1)$$

$$T(n) \lt 2^{n-1} \ast T(1) + (2^{n-1} - 1)$$

$$T(n) \lt 2^{n-1} + 2^{n-1} - 1$$

$$T(n) \lt 2^n - 1$$

## Fibonacci 2

1. Start with $$F(0)$$ and $$F(1)$$
2. Always keep the last two value, $$F(i-1)$$ and $$F(i-2)$$
3. Compute until $$F(i=n)$$

In this case, $$T(n) = n$$

This is called dynamic programming, when we save data that is frequently used.

How can we do better?

## Closed form of $$F(n)$$

$$F(n) = \frac{(golden\ ratio)^n - (golden\ ratio\ conjugate)^n}{\sqrt{5}}$$

$$T(n) = 2*(the\ cost\ of\ the\ power\ function)$$

The actual $$T(n)$$ for the recursive function is $$O((golden\ ratio)^n)$$

$$a^n = a \ast a \ast a \ast a \ast a \ast \dots \ast a$$

Which would be O(n) and worse than the dynamic programming version

$$a^n = a^{n/2} \ast a^{n/2}\ \text{if n is even}$$

$$a^n = a^{n/2} \ast a^{n/2} \ast a\ \text{if n is odd}$$

Next: Solve the power function analysis