A135981 Number of distinct prime factors of A135972(n).

0, 2, 2, 3, 2, 3, 2, 4, 3, 3, 4, 4, 5, 3, 4, 2, 6, 3, 3, 3, 6, 3, 6, 5, 4, 3, 4, 8, 2, 3, 4, 7, 2, 6, 3, 7, 6, 4, 3, 9, 2, 7, 5, 7, 3, 6, 6, 8, 4, 6, 2, 11, 3, 6, 7, 3, 8, 2, 7, 4, 9, 3, 12, 3, 5, 7, 7, 4, 7, 3, 9, 6, 5, 2, 12, 3, 5, 6, 10, 11, 5, 9, 3, 6, 5, 12, 2, 5, 8, 12

Offset: 2

Author

Artur Jasinski, Dec 09 2007

Keywords

• nonn

Examples

			A135972(3) = 15 = 3*5 which has a(3)=2 distinct prime factors.
		

Links

• Amiram Eldar, Table of n, a(n) for n = 2..1193

Crossrefs

Cf. A046051, A046053, A022307, A135972.

Programs

Mathematica

k = {}; Do[If[ ! PrimeQ[2^n - 1], c = FactorInteger[2^n - 1]; d = Length[c]; AppendTo[k, d]], {n, 1, 100}]; k

Formula

a(n) = A001221(A135972(n)) .

Extensions

Offset set to 2, definition shortened - R. J. Mathar, Oct 01 2009