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

1,3

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)

Links

Table of n, a(n) for n=1..82.

Example

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

Sequence in context: A321915 A321748 A263447 * A118923 A292745 A047010

Adjacent sequences: A180300 A180301 A180302 * A180304 A180305 A180306

Keyword

sign

Author

Carl R. White, Aug 25 2010

Status

approved