1,2

As n increases sum of a(n) from 1 to n / n tends to 2.66.

Pierre CAMI, Table of n, a(n) for n = 1..3000

(PARI) a(n) = {nb = 0; for (k=1, n, if (isprime((2^k-1)*2^n-1), nb++); ); return (nb); } \\ Michel Marcus, Mar 07 2013

Cf. A197117, A197118, A197119.

Sequence in context: A346801 A326375 A071470 * A104461 A308042 A335694

Adjacent sequences: A197113 A197114 A197115 * A197117 A197118 A197119

nonn

Pierre CAMI, Oct 11 2011

approved