A137990 - OEIS

A137990

Least prime p of the form c*3^n+1 with c not divisible by 3.

2, 7, 19, 109, 163, 487, 1459, 17497, 52489, 39367, 472393, 4960117, 5314411, 102036673, 19131877, 57395629, 86093443, 258280327, 3874204891, 23245229341, 90656394427, 585779779369, 251048476873, 9790890598009, 4518872583697

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OFFSET

0,1

COMMENTS

a(n) is also the least prime such that 3^(n+1), but not 3^(n+2), divides 2^(a(n)-1)-1.

LINKS

Table of n, a(n) for n=0..24.

Carlos Rivera, Puzzle 1129 OEIS A137990, The Prime Puzzles & Problems Connection.

EXAMPLE

a(8)=52489 because 52489=8*3^8+1 is prime and no smaller prime p has p-1 divisible by 3^8 but not 3^9.

CROSSREFS

Cf. A005109, A057775.

Sequence in context: A080873 A126162 A054423 * A056650 A182169 A373765

Adjacent sequences: A137987 A137988 A137989 * A137991 A137992 A137993

KEYWORD

easy,nonn

AUTHOR

Andrew V. Sutherland, May 01 2008

STATUS

approved