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A180303
a(n) = smallest number such that 3^n-2^a(n) is prime, or -1 if no such number exists.
2
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)
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
KEYWORD
sign
AUTHOR
Carl R. White, Aug 25 2010
STATUS
approved