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A181523 Greatest k < n such that 3^n - 3^k + 1 is prime or 0 if no such prime exists. 3
0, 1, 2, 2, 4, 5, 6, 4, 7, 9, 6, 10, 3, 10, 12, 0, 16, 17, 12, 17, 12, 3, 20, 22, 21, 17, 22, 0, 0, 22, 30, 29, 11, 31, 32, 18, 17, 35, 35, 37, 12, 25, 38, 14, 15, 12, 42, 37, 35, 42, 42, 36, 40, 52, 54, 16, 48, 57, 35, 58 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

LINKS

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

MAPLE

A181523 := proc(n) local p, k; for k from n-1 to 0 by -1 do p := 3^n-3^k+1 ;

if isprime(p) then return k; end if; end do: return 0 ; end proc:

seq(A181523(n), n=1..80) ; # R. J. Mathar, Jan 31 2011

PROG

(Sage)

A181523 = lambda n: next((k for k in reversed(IntegerRange(1, n)) if is_pseudoprime(3^n-3^k+1)), 0) # D. S. McNeil, Jan 31 2011

CROSSREFS

Cf. A181515.

Sequence in context: A137413 A010848 A133937 * A181537 A130498 A263433

Adjacent sequences:  A181520 A181521 A181522 * A181524 A181525 A181526

KEYWORD

nonn

AUTHOR

Pierre CAMI, Jan 29 2011

STATUS

approved

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Last modified June 18 18:09 EDT 2021. Contains 345120 sequences. (Running on oeis4.)