Fibonacci sequence

The Fibonacci Sequence is a sequence that appears often in nature. It is usually defined with:


 * $$a_0$$ = 0
 * $$a_1$$ = 1
 * $$a_i = a_{i-1}  a_{i-2}$$ for all $$i \geq 2$$.

The first few terms are: 0, 1, 1, 2, 3, 5, 8, 13, 21...

Properties
The $$N$$-th Fibonacci number can be calculated by a closed form formula: $$F_n = \frac{ ( 1  \sqrt{ 5 } )^n - ( 1 - \sqrt{ 5 } )^n }{ 2^n \sqrt{ 5 } }$$

If we rewrite the formula in the following way:

$$F_n = \frac{1}{\sqrt{5}} \cdot \left( \frac{\sqrt{ 5 } 1}{2} \right)^n  \frac{1}{\sqrt{5}} \cdot \left( \frac{\sqrt{ 5 }-1}{2} \right)^n$$

we can see that for $$n\to\infty$$ the second term goes to zero. In fact, it decreases with increasing $$n$$, and already for $$n=0$$ the second term is less than 0.5. Thus we can deduce $$F_n = {\rm round} \left( \frac{1}{\sqrt{5}} \cdot \left( \frac{\sqrt{ 5 } 1}{2} \right)^n \right) $$

Q-Matrix
Another interesting approach:

Consider an arbitrary vector $$v=(a,b)$$. When we multiply it by the matrix $$A = \left({0 ~ 1 \atop 1 ~ 1}\right) $$, we get the vector $$vA=(b,a b)$$. Thus if $$v=(F_n,F_{n 1})$$, then $$vA = (F_{n 1},F_n F_{n 1}) = (F_{n 1},F_{n 2})$$. In general, let $$v=(F_0,F_1)$$, then $$(\dots((v\underbrace{A)A)\dots)}_{n{\rm~times}} = (F_n,F_{n 1})$$. As matrix multiplication is associative, we may rewrite the left side of the equation to get $$vA^n$$. The matrix $$A^n$$ can be computed in time logarithmic in $$n$$ using repeated squaring.

Pseudocode
This function gives the $$nth$$ Fibonacci number. function fib(n) integer a = 1 integer b = 0 integer t

for i from 1 to n       t = a   b        b = a        a = t    return a