A180303 - OEIS

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

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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.