281!/281# ± 1 are twin primes. Note that 281 is a prime of form n!3 + 1. [Luhn]
Wilfred Whiteside of Houston, Texas, discovered the following 7-by-7 array in which 281 primes can be found:
3 1 3 7 3 3 9
9 9 2 3 3 3 3
6 9 7 7 8 9 4
7 6 1 5 9 1 9
7 7 3 4 2 1 1
9 9 4 7 9 3 9
3 3 7 1 9 9 9.
The largest prime p such that (1!+2!+3!+4!+ ... +p!) - 2 is
prime. [Capelle]
281 is the sum of consecutive primes up to 43. If we add consecutive primes up to 281, it too is prime (7699). Note that there are no other such primes with this behavior between 281 and 7699. [Poo Sung]
The largest prime partial sum of number of irreducible
diagrams (in the sense of perturbation expansion in quantum
field theory: spinor case in 4 spacetime dimensions) with
2n nodes. [Post]
281 = 9*8 + 7*6 + 5*4 + 3*2 + 1 + 2*3 + 4*5 + 6*7 + 8*9. [Silva]
The number of strong primes and the number of weak primes are equal for the first time at 281 (the 60th prime). [Honaker]
(There are 2 curios for this number that have not yet been approved by an editor.)
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