A123675 a(n) = number of primes of the form 2^n - 5^k.

0, 1, 2, 1, 2, 1, 3, 2, 1, 1, 1, 2, 2, 1, 1, 0, 2, 2, 1, 1, 1, 1, 0, 2, 1, 4, 2, 3, 1, 0, 2, 2, 4, 1, 0, 3, 2, 2, 3, 0, 0, 2, 0, 3, 0, 1, 0, 1, 1, 2, 0, 3, 1, 0, 1, 2, 1, 1, 3, 0, 3, 2, 2, 2, 0, 3, 0, 2, 1, 0, 2, 1, 2, 2, 1, 2, 1, 1, 1, 1, 2, 1, 0, 0, 0, 0, 0, 2, 1, 1, 1, 2, 1, 3, 1, 0, 2, 2, 4, 2, 2, 1, 0, 3, 0

Offset

1,3

Comments

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

Links

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

Mathematica

Table[Length[Select[Range[0, Floor[Log[5, 2^n]]], PrimeQ[2^n-5^# ]&]], {n, 1, 150}]

Crossrefs

Sequence in context: A154819 A104145 A230981 * A356226 A322872 A356227

Adjacent sequences: A123672 A123673 A123674 * A123676 A123677 A123678

Keyword

nonn

Author

Alexander Adamchuk, Nov 17 2006

Status

approved