The Fibonacci sequence is defined as Fn = Fn-1 + Fn-2, with F0 = 0 and F1 = 1. The first few numbers are: 0, 1, 1, 2, 3, 5, 8 and 13.
An interesting way to visualise Fibonacci numbers is to start off with two 1x1 squares, and then combine their side lengths to draw a new square, 2x2. This can be continued ad infinitum to form the Fibonacci spiral.
The ratio of successive Fibonacci numbers approaches the golden ratio, φ, as n increases.
Fn / Fn-1 =