A366931 Least k such that 3*n*k+1 is a prime.

2, 1, 2, 1, 2, 1, 2, 3, 4, 1, 2, 1, 2, 1, 4, 2, 2, 2, 4, 1, 2, 1, 2, 1, 2, 1, 2, 4, 4, 2, 4, 1, 2, 1, 2, 1, 2, 2, 8, 2, 6, 1, 8, 3, 2, 1, 2, 3, 6, 1, 2, 1, 20, 1, 2, 2, 12, 2, 4, 1, 2, 2, 2, 1, 6, 1, 8, 2, 4, 1, 4, 2, 2, 1, 8, 1, 2, 4, 6, 1, 2, 3, 2, 3, 4, 4

Offset: 1

Author

Robert Price, Dec 17 2023

Keywords

• nonn

Links

• Robert Price, Table of n, a(n) for n = 1..5000

Crossrefs

Cf. A016014, A024892.

A070847 lists the corresponding primes.

Programs

Mathematica

A366931 = {};
Do[k=1; While[!PrimeQ[3 n k+1], k++]; AppendTo[A366931 ,k], {n,85}];
A366931
lkp[n_]:=Module[{k=1},While[!PrimeQ[3n*k+1],k++];k]; Array[lkp,90] (* Harvey P. Dale, Jan 09 2025 *)

PARI

a(n) = my(k=1); while (!isprime(3*n*k+1), k++); k; \\ Michel Marcus, Dec 17 2023