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

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 A170869

Adjacent sequences: A137987 A137988 A137989 * A137991 A137992 A137993

Keyword

easy,nonn

Author

Andrew V. Sutherland, May 01 2008

Status

approved