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 A230165 #19 Feb 14 2025 08:26:30 %S A230165 6,15,42,47058 %N A230165 Numbers k such that k = sigma(k'), where k' is the arithmetic derivative of k. %C A230165 a(5) > 10^10. - _Giovanni Resta_, Oct 14 2013 %C A230165 Terms of this sequence and A342021 group into pairs (m, m'), where m is a term of this sequence and m' is a term of A342021. - _Max Alekseyev_, Feb 13 2025 %F A230165 a(n) = sigma(A342021(n)). - _Max Alekseyev_, Feb 13 2025 %e A230165 Arithmetic derivative of 15 is 8 and sigma(8) = 15. %p A230165 with(numtheory); P:=proc(q) local n; for n from 1 to q do %p A230165 if n=sigma(n*add(op(2,p)/op(1,p),p=ifactors(n)[2])) then print(n); %p A230165 fi; od; end: P(10^9); %Y A230165 Cf. A003415, A229342, A230164, A342021. %K A230165 nonn,more %O A230165 1,1 %A A230165 _Paolo P. Lava_, Oct 14 2013