Definition:Fibonacci Number/Negative

Definition
The definition of Fibonacci numbers for negative integers is an extension of the definition for positive integers:
 * $F_n = \begin{cases} 0 & : n = 0 \\

1 & : n = 1 \\ F_n = F_{n + 2} - F_{n + 1} & : n < 0 \end{cases}$ for all $n \in \Z$.