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A124874 Number of distinct primes dividing A007408(n).

%I A124874 #10 Feb 09 2020 08:40:08
%S A124874 0,1,1,3,2,3,3,2,3,4,3,2,3,2,1,3,2,4,1,3,2,5,2,4,2,1,3,4,3,5,1,2,3,2,
%T A124874 3,8,4,4,5,4,8,6,7,2,4,6,3,2,4,3,2,4,3,4,2,8,3,3,4,4,3,3,7,2,5,9,4,2,
%U A124874 6,7,3,5,6,4,3,6,4,6,4,4,3,3,8,5,7,4,5
%N A124874 Number of distinct primes dividing A007408(n).
%H A124874 Amiram Eldar, <a href="/A124874/b124874.txt">Table of n, a(n) for n = 1..106</a>
%F A124874 a(n) = A001221(A007408(n)).
%e A124874 a(1) = 0 since no prime divides A007408(1) = 1,
%e A124874 a(2) = 1 since 3 is the only prime dividing A007408(2) = 9,
%e A124874 a(3) = 1 since A007408(3) = 251 is prime,
%e A124874 a(4) = 3 since A007408(4) = 5 * 11 * 37,
%e A124874 a(6) = 3 since 7, 11, 53 are the only primes dividing A007408(6) = 7^2 * 11 * 53.
%p A124874 seq( (nops@op)(2,(ifactors@A007408)(n)),n=1..25);
%t A124874 Table[PrimeNu[Numerator[Sum[1/k^3, {k, 1, n}]]], {n, 1, 50}] (* _Amiram Eldar_, Feb 09 2020 *)
%Y A124874 Cf. A001221, A007408, A124876, A124877.
%K A124874 nonn
%O A124874 1,4
%A A124874 _M. F. Hasler_, Nov 11 2006
%E A124874 More terms from _Amiram Eldar_, Feb 09 2020