The smallest number with 2 prime factors that do not divide 10.
221 - 21 is prime. [Brown]
Blackjack primes are separated by exactly 21 consecutive composite numbers.
Note that the pair {1129, 1151} is the smallest example.
The smallest prime of the form n21 + 21 begins with a decimal expansion of  = 3.141....
Phi(21) = 12. [Honaker]
The 21st set of blackjack primes (primes with a gap of 21) is 11329 and 11351. [Fougeron]
The smallest triangular number whose sum of aliquot divisors is prime. [Gupta]
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]
The number of known positive integers which cannot be
written as p * q + r, where p, q, r are three distinct
primes. [Capelle]
The smallest semiprime that is a product of distinct Mersenne primes.
21 repeated twenty-one times, following 1, forms a smoothly undulating palindromic prime. [Silva]
21 followed by its reversal, plus/minus one, are twin
primes. The first integer with that property. [Silva]
21 = (2*21) + (1*21). [Arabi]
(There are 4 curios for this number that have not yet been approved by an editor.) To link to this page use http://primes.utm.edu/curios/page.php?number_id=379
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