A023589 a(n) is the number of distinct prime divisors of 2*prime(n)+1.

1, 1, 1, 2, 1, 1, 2, 2, 1, 1, 2, 2, 1, 2, 2, 1, 2, 2, 2, 2, 2, 2, 1, 1, 3, 2, 2, 2, 2, 1, 3, 1, 2, 2, 2, 2, 3, 2, 2, 1, 1, 2, 1, 2, 2, 3, 2, 2, 3, 2, 1, 1, 3, 1, 2, 2, 2, 2, 3, 1, 2, 1, 3, 2, 3, 2, 3, 2, 2, 2, 2, 1, 3, 2, 3, 2, 2, 3, 2, 3, 1, 2, 1, 2, 2, 1, 2, 3, 2, 2, 3, 2, 3, 1, 2, 2, 1, 2, 2

Offset: 1

Author

Clark Kimberling

Keywords

• nonn

Links

• Michael De Vlieger, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A001221, A072055.

Programs

Mathematica

Array[PrimeNu[2 Prime[#] + 1] &, 99] (* Michael De Vlieger, Jul 09 2022 *)

PARI

a(n) = omega(2*prime(n)+1) \\ Hugo Pfoertner, Jul 09 2022