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A197116
Number of k such that (2^k-1)*2^n-1 is prime with 0 < k < n+1.
4
0, 2, 2, 2, 4, 1, 3, 1, 2, 2, 5, 1, 4, 2, 1, 2, 4, 3, 3, 1, 2, 1, 4, 1, 1, 1, 0, 2, 3, 1, 6, 1, 3, 1, 2, 3, 3, 3, 3, 1, 7, 3, 2, 1, 4, 2, 3, 1, 3, 1, 2, 2, 3, 1, 4, 3, 3, 1, 3, 0, 4, 0, 3, 3, 3, 2, 1, 2, 0, 3, 1, 2, 4, 4, 3, 1, 4, 2, 3, 3, 3, 2, 3, 6, 0, 4, 4, 2, 3
OFFSET
1,2
COMMENTS
As n increases sum of a(n) from 1 to n / n tends to 2.66.
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