OFFSET
1,2
COMMENTS
As n increases sum of a(n) from 1 to n / n tends to 2.66.
LINKS
Pierre CAMI, Table of n, a(n) for n = 1..3000
MATHEMATICA
Table[Count[2^n (2^Range[n]-1)-1, _?PrimeQ], {n, 90}] (* Harvey P. Dale, Sep 16 2024 *)
PROG
(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
CROSSREFS
KEYWORD
nonn
AUTHOR
Pierre CAMI, Oct 11 2011
STATUS
approved