This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A054991 #23 Mar 06 2022 23:04:50
%S A054991 0,0,1,1,2,1,1,2,3,2,4,1,2,1,5,2,3,3,3,2,4,3,2,2,3,2,2,4,5,1,3,1,1,2,
%T A054991 3,2,5,1,4,2,4,4,7,4,5,5,2,4,3,2,5,5,4,6,6,5,6,5,2,3,4,4,5,4,6,4,7,2,
%U A054991 6,5,5,3,4,5,7,3,5,4,2,4,4,4,4,6,2,3,4
%N A054991 Number of prime divisors of n! - 1 (counted with multiplicity).
%C A054991 The series is related to the product of primes and the "proof" of the existence of infinite many prime twins.
%H A054991 Amiram Eldar, <a href="/A054991/b054991.txt">Table of n, a(n) for n = 1..135</a>
%H A054991 R. G. Wilson v, <a href="/A038507/a038507.txt">Explicit factorizations</a>
%H A054991 Hisanori Mishima, <a href="http://www.asahi-net.or.jp/~KC2H-MSM/mathland/matha1/matha103.htm">Factorizations of many number sequences</a>
%H A054991 Hisanori Mishima, <a href="http://www.asahi-net.or.jp/~KC2H-MSM/mathland/matha1/matha105.htm">Factorizations of many number sequences</a>
%e A054991 a(2)=0 because 2! - 1 = 1 (and this is not a prime number) a(5)=2 because 5! -1 = 119 = 7 * 17
%t A054991 a[q_] := Module[{x, n}, x=FactorInteger[q!-1]; n=Length[x]; Sum[Table[x[[i]][[2]], {i, n}][[j]], {j, n}]]
%t A054991 A054991[n_Integer] := PrimeOmega[n! - 1]; A054991[1] = 0; Table[A054991[n], {n,2,100}] (* _Vladimir Joseph Stephan Orlovsky_, Jul 22 2011 *)
%Y A054991 Cf. A033312, A054988, A054989, A054990, A054992.
%K A054991 nonn
%O A054991 1,5
%A A054991 Arne Ring (arne.ring(AT)epost.de), May 30 2000
%E A054991 More terms from _Robert G. Wilson v_, Mar 24 2001
%E A054991 More terms from _Amiram Eldar_, Oct 03 2019