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 A123391 #19 Sep 29 2023 04:01:50
%S A123391 0,0,0,2,0,0,0,3,2,0,0,2,0,0,0,0,0,2,0,2,0,0,0,3,2,0,3,2,0,0,0,5,0,0,
%T A123391 0,4,0,0,0,3,0,0,0,2,2,0,0,0,2,2,0,2,0,3,0,3,0,0,0,2,0,0,2,0,0,0,0,2,
%U A123391 0,0,0,5,0,0,2,2,0,0,0,0,0,0,0,2,0,0,0,3,0,2,0,2,0,0,0,5,0,2,2,4,0,0,0,3,0
%N A123391 a(n) = sum of exponents that are primes in the prime factorization of n.
%H A123391 Antti Karttunen, <a href="/A123391/b123391.txt">Table of n, a(n) for n = 1..10000</a>
%H A123391 <a href="/index/Eu#epf">Index entries for sequences computed from exponents in factorization of n</a>.
%F A123391 Additive with a(p^e) = A010051(e)*e. - _Antti Karttunen_, Jul 19 2017
%F A123391 Asymptotic mean: Limit_{m->oo} (1/m) * Sum_{k=1..m} a(k) = Sum_{p prime} p*(P(p)-P(p+1)) = 0.97487020987790163735..., where P(s) is the prime zeta function. - _Amiram Eldar_, Sep 29 2023
%e A123391 36 = 2^2*3^2. Both exponents in this prime factorization are primes. So a(36) = 2+2 = 4.
%t A123391 f[n_] := Plus @@ Select[Last /@ FactorInteger[n], PrimeQ];Table[f[n], {n, 120}] (* _Ray Chandler_, Nov 11 2006*)
%o A123391 (PARI) {m=105; for(n=1, m, v=factor(n)[,2]; s=0; for(j=1, #v, if(isprime(v[j]), s=s+v[j])); print1(s,","))} \\ _Klaus Brockhaus_, Nov 14 2006
%o A123391 (PARI) A123391(n) = vecsum(apply(e -> isprime(e)*e, factorint(n)[, 2])); \\ _Antti Karttunen_, Jul 19 2017
%Y A123391 Cf. A010051, A101436, A125030, A125071, A125073.
%K A123391 nonn,easy
%O A123391 1,4
%A A123391 _Leroy Quet_, Nov 10 2006
%E A123391 Extended by _Ray Chandler_ and _Klaus Brockhaus_, Nov 11 2006