OFFSET
1,4
COMMENTS
a(21) > 4200. - Robert G. Wilson v
a(21) > 12000, a(22)=6669, a(23) > 10000, a(24)=5970. - Ray G. Opao, Apr 15 2004
EXAMPLE
a(4) = 12: 2(12)(2^7-1)(2^13-1)-1 = 24(127)(8191)-1 = 24966167, which is prime.
MATHEMATICA
p = { (* the list of Mersenne exponents in A000043 *) }; f[n_] := Block[{k = 1, q = 2^p[[n]] - 1, r = 2^p[[n + 1]] - 1}, While[ !PrimeQ[2k*q*r - 1], k++ ]; k]; Do[ Print[ f[n]], {n, 25}] (* Robert G. Wilson v, Apr 10 2004 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Ray G. Opao, Apr 08 2004
STATUS
approved