A181515 Greatest k < n such that 3^n - 3^k - 1 is prime, or 0 if no such prime exists.

0, 1, 2, 3, 2, 4, 0, 7, 8, 4, 7, 10, 12, 7, 10, 12, 13, 15, 17, 17, 20, 16, 15, 23, 17, 8, 3, 27, 24, 22, 19, 23, 25, 23, 30, 35, 13, 31, 22, 30, 21, 39, 17, 37, 32, 40, 42, 45, 45, 43, 45, 50, 50, 48, 34, 53, 56, 0, 54, 25

Offset

1,3

Links

Pierre CAMI, Table of n, a(n) for n = 1..2000

Maple

A181515 := proc(n) local p, k; for k from n-1 to 1 by -1 do p := 3^n-3^k-1 ; if isprime(p) then return k; end if; end do: return 0 ; end proc: # R. J. Mathar, Jan 30 2011

Mathematica

Table[Max[Flatten[Position[3^n-3^Range[n-1]-1, _?PrimeQ]]], {n, 60}]/.-\[Infinity]->0  (* Harvey P. Dale, Feb 12 2011 *)

Crossrefs

Sequence in context: A167504 A286013 A208083 * A183045 A060847 A079528

Adjacent sequences: A181512 A181513 A181514 * A181516 A181517 A181518

Keyword

nonn

Author

Pierre CAMI, Jan 29 2011

Status

approved