A050921 -id:A050921 - cp's OEIS frontend

A178948 a(n) = smallest m>=0 such that n*(3^m)-1 is prime, or -1 if no such prime exists.

1, 1, 0, 0, -1, 0, -1, 0, -1, 1, -1, 0, -1, 0, -1, 1, -1, 0, -1, 0, -1, 2, -1, 0, -1, 2, -1, 1, -1, 0, -1, 0, -1, 1, -1, 1, -1, 0, -1, 2, -1, 0, -1, 0, -1, 1, -1, 0, -1, 1, -1, 2, -1, 0, -1, 1, -1, 1, -1, 0, -1, 0, -1, 1, -1, 1, -1, 0, -1, 3, -1, 0, -1, 0, -1, 1, -1, 1, -1, 0, -1, 3, -1, 0, -1, 1, -1, 1, -1, 0

Offset: 1

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Jonathan Vos Post, Dec 31 2010

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For odd n>=5, a(n)=-1 because then n*3^m is odd, therefore n*3^m-1 even and not prime. - R. J. Mathar, Jan 03 2011

			a(1) = 1 because 1*(3^1) - 1 = 2 is prime.
a(5) = -1 because 5*(3^m) - 1 is even and >=4.
a(22) = 2 because 22*(3^2)-1 = 197 is prime.

Cf. A038699, A050921.

cp's OEIS Frontend

A178948 a(n) = smallest m>=0 such that n*(3^m)-1 is prime, or -1 if no such prime exists.

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