A359180 Numbers k such that k!^2 / 2 + 1 is prime.

2, 3, 6, 18, 19, 82, 1298, 3139, 3687, 4637

Offset: 1

Author

Arsen Vardanyan, Dec 18 2022

Keywords

• nonn
• hard
• more

Examples

			3!^2 / 2 + 1 = 6^2/2 + 1 = 19, a prime number, so 3 is a term.
		

Crossrefs

Cf. A002981, A046029, A358805, A358878.

Programs

PARI

isok(k) = (k>1) && isprime(k!^2 / 2 + 1); \\ Michel Marcus, Jan 15 2023

Extensions

a(7) from Michael S. Branicky, Dec 18 2022

a(8)-a(10) from Michael S. Branicky, Apr 10 2023