A382447 Number of positive k <= n such that k*2^n - 1 is prime.

0, 2, 2, 2, 2, 3, 2, 1, 1, 3, 3, 2, 3, 2, 2, 4, 6, 3, 1, 3, 3, 0, 1, 0, 1, 1, 2, 3, 2, 3, 4, 2, 2, 1, 5, 2, 4, 2, 1, 3, 4, 3, 4, 2, 2, 3, 2, 3, 2, 3, 3, 3, 4, 5, 2, 2, 3, 1, 3, 3, 3, 4, 3, 1, 0, 1, 2, 1, 4, 3, 3, 5, 3, 3, 6, 2, 3, 3, 3, 2, 3, 1, 1, 1, 3, 1, 2, 2, 2, 2, 3, 3, 2, 3, 2, 3, 2, 3, 3, 2

Offset: 1

Author

Juri-Stepan Gerasimov, Mar 26 2025

Keywords

• nonn

Crossrefs

Cf. A002234, A003261, A382119.

Cf. A061411 (indices of 0s), A061414 (indices of 1s).

Programs

Magma

[#[k: k in [1..n] | IsPrime(k*2^n-1)]: n in [1..100]];

Mathematica

a[n_]:=Length[Select[Range[n],PrimeQ[#*2^n-1] &]]; Array[a,100] (* Stefano Spezia, Mar 26 2025 *)