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A cinematic twilight landscape with glowing prime numbers 2, 3, 5, 7, 11, 13, …,  and 563 placed along a rocky ridge, while a dotted trail continues into a wide desert under a star-filled sky.

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MATHEMATICAL MYSTERIES

We Know Only Three Wilson Primes

Every prime satisfies Wilson’s theorem. Only three known primes go one step further — and nobody knows if a fourth exists.

13 min readOct 15, 2026

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Prime numbers are already unusual. They become less frequent as numbers grow, resist simple patterns, and continue to generate questions that look elementary but remain unresolved.

Yet hidden among the primes is a far rarer species, the Wilson primes.

Only three are known:

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5, 13, 563

What makes them special can be stated surprisingly simply.

For every prime p, Wilson’s theorem guarantees that (p − 1)! + 1 is divisible by p:

p divides (p−1)! + 1

But Wilson primes satisfy a rarer condition: the same number is divisible by p²:

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Structural engineer, professor and science communicator exploring engineering, mathematics, AI and science through clear, useful and engaging stories.