A303737 Numbers k such that (2*k)! + k! - 1 is prime.
1, 4, 18, 49, 60, 82, 321, 6328
Offset: 1
Examples
1 is a term because (2*1)! + 1! - 1 = 2 which is a prime. 4 is a term because (2*4)! + 4! - 1 = 40343 which is a prime.
Programs
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Maple
select(k->isprime(factorial(2*k)+factorial(k)-1),[$1..1000]);
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PARI
isok(k) = isprime((2*k)! + k! - 1); \\ Michel Marcus, May 28 2018
Extensions
a(8) from Giovanni Resta, Jun 07 2018
Comments