Fibonacci number checker
Check whether a number is part of the Fibonacci sequence (0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55...).
Number
Fibonacci test
A number n is Fibonacci if and only if 5n²+4 or 5n²−4 is a perfect square. 13: 5(169)+4 = 849 (not square), 5(169)−4 = 841 = 29² ✓ → 13 is Fibonacci.
Fibonacci numbers
0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765...
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