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

%I #31 Apr 11 2023 03:47:30

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

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

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

%o (PARI) isok(k) = (k>1) && isprime(k!^2 / 2 + 1); \\ _Michel Marcus_, Jan 15 2023

%Y Cf. A002981, A046029, A358805, A358878.

%K nonn,hard,more

%O 1,1

%A _Arsen Vardanyan_, Dec 18 2022

%E a(7) from _Michael S. Branicky_, Dec 18 2022

%E a(8)-a(10) from _Michael S. Branicky_, Apr 10 2023