Fibonacci

Returns the desired Fibonacci number. Fibonacci numbers are the numbers in the Fibonacci sequence, characterized by the fact that every number after the first two is the sum of the two preceding ones: $F\left(n\right)=F(n-1)+F(n-2)$. The first two numbers are defined as 1: $F\left(1\right)=1;F\left(2\right)=1$.

If $n$ is negative, the Fibonacci numbers are defined as $F(n-2)=F\left(n\right)-F(n-1)$.

fibonacci(Integer)

Description

Given an integer $n$, returns the $n$-th Fibonacci number: $F\left(n\right)$.