A180303 a(n) = smallest number such that 3^n-2^a(n) is prime, or -1 if no such number exists.

0, 1, 2, 1, 1, 1, 3, 3, 1, 7, 4, 11, 6, 3, 4, 9, 9, 5, 6, 3, 2, 1, 7, 35, 12, 29, 10, 13, 6, 11, 2, 21, 8, 27, 11, -1, 1, 17, 10, -1, 1, 37, 8, 9, 16, 61, 23, 23, 17, 27, 4, 7, 2, 7, 7, 39, 58, 81, 30, 17, 60, 3, 8, 13, 18, -1, 20, 101, 4, 73, 27, 17, 2, 17, 19, 13, 41, 53, 44, 111, 34, 13

Offset: 1

Author

Carl R. White, Aug 25 2010

Keywords

• sign

Comments

From Carl R. White, Oct 23 2010: (Start)

Entries where a(n) = 1 can be found in A014224.

Entries where a(n) = -1 can be found in A181484. (End)

Examples

			3^1-2^0 = 2, so a(1)=0; no other terms are zero.
3^11-2^1, 3^11-2^2, 3^11-2^3 are all nonprime, but 3^11-2^4 = 177131 which is prime so a(11) = 4.
a(36) is -1 (the placeholder value) because nonprimes are obtained when any power of two is subtracted from 3^36.
		

Crossrefs

Cf. A013604.

Cf. A014224, A181483, A181484. - Carl R. White, Oct 23 2010