A377365 a(n) = least k such that 2n*5^k+1 is prime, or 0 if no prime is reached.

1, 2, 1, 1, 2, 1, 1, 2, 3, 1, 4, 2, 1, 2, 1, 3, 8, 1, 1, 1036, 1, 3, 2, 1, 1, 2, 1, 1, 2, 4, 1, 2, 1, 3, 6, 2, 257, 2, 2, 1, 40, 1, 1, 4, 2, 1, 2, 10, 1, 4, 2, 1, 6, 1, 3, 2, 1, 15, 4, 1, 79, 48, 1, 1, 2, 1, 5, 6, 1, 1, 6, 4, 3, 2, 1, 1, 2, 3, 3, 2, 1, 1, 6

Offset: 1

Author

Clark Kimberling, Oct 31 2024

Keywords

• nonn

Examples

			a(20) = 1036 because 40*5^k+1 is prime for k=1036 and not prime for k=1..1035.
		

Crossrefs

Cf. A000040, A050412, A354747, A377364, A377366, A377367.

Programs

Mathematica

f[n_, k_] := 2 n*5^k + 1;
s[n_] := Select[Range[5000], PrimeQ[f[n, #]] &, 1];
Flatten[Table[s[n], {n, 1, 500}]]