Pisano period

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In mathematics, the nth Pisano period, written π(n), is the period with which the sequence of Fibonacci numbers, modulo n repeats. For example, the Fibonacci numbers mod 3 are 0, 1, 1, 2, 0, 2, 2, 1, 0, 1, 1, etc., with the first eight numbers repeating, so π(3) = 8.

The first Pisano periods (sequence A001175 in OEIS) for n = 1, 2, ... are:

1, 3, 8, 6, 20, 24, 16, 12, 24, 60, 10, 24, 28, 48, 40, 24, 36, 24, 18, 60, 16, 30, 48, 24, 100 ...

It has been observed that zero occurs only once, twice or four times throughout any Pisano sequence. Also, it can be proven that

π(n) ≤ 6n,

and that the equality applies infinitely often (whenever n/2 equals a power of 5).

Pisano periods are named after Leonardo Pisano, better known as Fibonacci.

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