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 A073721 #18 Mar 18 2025 07:29:45
%S A073721 2,3,5,6,7,14,15,21,29,38,44,57,66,78,92,94,95,106,108,114,116,118,
%T A073721 120,133,154,174,177,182,188,232,255,300,304,305,349,351,359,413,417,
%U A073721 418,468,488,506,526,595,615,629,688,872,945,954,1001,1002,1006,1011,1018
%N A073721 Numbers k such that PrimePi(k) divides sigma(k).
%H A073721 Amiram Eldar, <a href="/A073721/b073721.txt">Table of n, a(n) for n = 1..1600</a>
%F A073721 {k : A000203(k)/A000720(k) is an integer}.
%t A073721 Do[s=Mod[DivisorSigma[1, n], PrimePi[n]]; If[s==0, Print[n]], {n, 1, 2500}]
%t A073721 Select[Range[2,1200],Divisible[DivisorSigma[1,#],PrimePi[#]]&] (* _Harvey P. Dale_, Jan 31 2014 *)
%o A073721 (PARI) isok(k) = k > 1 && !(sigma(k) % primepi(k)); \\ _Amiram Eldar_, Mar 18 2025
%o A073721 (PARI) list(lim) = {my(prv = 2, i = 0); forprime(p = 3, lim, i++; for(k = prv, p-1, if(!(sigma(k) % i), print1(k, ", "))); prv = p);} \\ _Amiram Eldar_, Mar 18 2025
%Y A073721 Cf. A000203, A000720, A073722, A073723.
%K A073721 nonn
%O A073721 1,1
%A A073721 _Labos Elemer_, Aug 05 2002