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 A090999 #8 Jun 20 2025 03:28:54
%S A090999 1,15,21,33,23069,42029,148031,209911,3271099
%N A090999 Numbers k such that sigma(k)-k = pi(sigma(k)).
%t A090999 Do[If[DivisorSigma[1, n]-n==PrimePi[DivisorSigma[1, n]], Print[n]], {n, 1, 10000}]
%o A090999 (PARI) isok(k) = {my(s = sigma(k)); s - k == primepi(s);} \\ _Amiram Eldar_, Jun 20 2025
%Y A090999 Cf. A000203 (sigma), A000720 (pi), A001065, A070803.
%K A090999 nonn,more
%O A090999 1,2
%A A090999 _Naohiro Nomoto_, Mar 01 2004
%E A090999 a(9) from _Amiram Eldar_, Jun 20 2025