A123674 a(n) = number of primes of the form 2^n - 3^k.

0, 1, 2, 2, 4, 2, 3, 2, 4, 2, 2, 2, 3, 3, 1, 2, 4, 0, 3, 4, 4, 3, 3, 3, 0, 1, 1, 0, 2, 1, 1, 1, 3, 2, 3, 2, 2, 0, 1, 2, 2, 3, 0, 0, 4, 4, 3, 2, 5, 4, 4, 0, 0, 0, 1, 1, 4, 5, 2, 4, 3, 3, 0, 1, 1, 2, 5, 0, 1, 1, 4, 3, 1, 0, 1, 1, 3, 2, 3, 0, 2, 4, 2, 1, 2, 2, 3, 0, 7, 2, 4, 4, 2, 2, 2, 3, 5, 0, 3, 1, 1, 1, 3, 3, 2

Offset

1,3

Comments

a(1) = 0 because there are no prime numbers of the form 2^1 - 3^k. a(2) = 1 because the only prime of the form 2^2 - 3^k is 2^2 - 3^0 = 3. a(3) = because there are two primes of the form 2^3 - 3^k: 2^3 - 3^0 = 7 and 2^3 - 3^1 = 5.

Links

G. C. Greubel, Table of n, a(n) for n = 1..1000

Mathematica

Table[Length[Select[Range[0, Floor[Log[3, 2^n]]], PrimeQ[2^n-3^# ]&]], {n, 1, 200}]

Crossrefs

Sequence in context: A102640 A332581 A328059 * A363219 A371531 A238745

Adjacent sequences: A123671 A123672 A123673 * A123675 A123676 A123677

Keyword

nonn

Author

Alexander Adamchuk, Nov 17 2006

Status

approved