Prime Curios!: 21

21
(just those submitted by Firoozbakht)

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21 is the only number m such that m =

(

(m)!₂))). [Firoozbakht]

21 is the only known multidigit number m such that both numbers m * m! - prime(m) and m * m! + prime(m) are primes. [Firoozbakht]