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 A039780 #15 Jul 10 2024 20:04:46 %S A039780 5,25,1176,1625,7385,18633,21726,30276,32923,36905,46025,50809,55449, %T A039780 60726,89257,102425,151657,185745,278226,289961,301017,319145,373176, %U A039780 394121,459513,779817,815676,827926,1053019,1055719,1170153,1399123,1657865,1663209,1667326 %N A039780 Numbers k such that phi(k) is equal to A008473(k-1). %e A039780 phi(1176)=336, 1175=5^2*47^1, (5+2)*(47+1)=336. %o A039780 (PARI) b(n) = my(f = factor(n)); for (k=1, #f~, f[k, 1] = f[k, 1] + f[k, 2]; f[k, 2] = 1; ); factorback(f); \\ A008473 %o A039780 isok(k) = (k>1) && (eulerphi(k) == b(k-1)); \\ _Michel Marcus_, Feb 25 2021 %Y A039780 Cf. A000010, A008473. %Y A039780 Cf. A039779, A039781. %K A039780 nonn %O A039780 1,1 %A A039780 _Olivier Gérard_ %E A039780 Title corrected and more terms from _Sean A. Irvine_, Feb 24 2021