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21 (another Prime Pages' Curiosity) |
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Single Curio View: (Seek other curios for this number) 21 is the only known multidigit number m such that both numbers m * m! - prime(m) and m * m! + prime(m) are primes. [Firoozbakht]
Submitted: 2004-09-11 03:35:00; Last Modified: 2008-09-22 21:22:12.
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