Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #10 Jun 22 2020 04:48:35
%S 1,1,1,1,1,2,1,1,1,2,2,1,1,2,1,1,1,2,3,1,1,1,1,3,1,1,2,3,2,2,1,2,1,2,
%T 1,2,2,1,2,1,1,4,1,2,1,1,3,2,1,3,3,1,2,2,1,3,2,1,1,4,1,1,1,2,4,1,2,1,
%U 1,1,2,2,2,3,3,2,3,1,3,3,1,2,3,1,2,2,1,1,4,1,1,1,2,1,4,1,1,3,4,1,1,2
%N Number of semiprime divisors of n-th composite number.
%H Robert Israel, <a href="/A088737/b088737.txt">Table of n, a(n) for n = 1..10000</a>
%F a(n) = A056170(n) + A001221(n)*(A001221(n)-1)/2.
%p f:= proc(n) local p, t;
%p p:= numtheory:-factorset(n);
%p t:= nops(p);
%p t*(t-1)/2 + nops(select(m-> n mod m^2=0, p))
%p end proc:
%p map(f, remove(isprime,[$4..200])); # _Robert Israel_, May 21 2020
%t a[n_] := Count[FactorInteger[n], {_, k_ /; k>1}]+PrimeNu[n](PrimeNu[n]-1)/2;
%t a /@ Select[Range[200], CompositeQ] (* _Jean-François Alcover_, Jun 22 2020 *)
%Y Cf. A088738, A002808, A001358.
%K nonn
%O 1,6
%A _Reinhard Zumkeller_, Oct 12 2003