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 A074786 #26 Apr 29 2025 04:43:52
%S A074786 1,99,1080,1836,4743,5670,7644,8307,14384,14784,15225,15824,16065,
%T A074786 20300,21584,25704,29760,34544,46816,71568,94240,128412,169290,264160,
%U A074786 266266,312480,331731,364941,404550,445050,454575,458052,479655,497781
%N A074786 Numbers k such that phi(k) = Sum_{d|k} core(d) where core(x) is the squarefree part of x (A007913).
%C A074786 Numbers k such that A000010(k) = A069088(k). - _Amiram Eldar_, Apr 28 2025
%H A074786 Amiram Eldar, <a href="/A074786/b074786.txt">Table of n, a(n) for n = 1..1000</a> (terms 1..500 from Donovan Johnson)
%t A074786 f[p_, e_] := If[OddQ[e], (p+1)*(e+1)/2, (p+1)*e/2 + 1] / ((p-1)*p^(e-1)); q[1] = True; q[n_] := Times @@ f @@@ FactorInteger[n] == 1; Select[Range[500000], q] (* _Amiram Eldar_, Apr 28 2025 *)
%o A074786 (PARI) isok(n) = eulerphi(n) == sumdiv(n, d, core(d)); \\ _Michel Marcus_, Aug 09 2013
%o A074786 (PARI) isok(k) = {my(f = factor(k)); prod(i = 1, #f~, p = f[i, 1]; e = f[i, 2]; if(e % 2, (p+1)*(e+1)/2, (p+1)*e/2 + 1) / ((p-1)*p^(e-1))) == 1;} \\ _Amiram Eldar_, Apr 28 2025
%Y A074786 Cf. A000010, A007913, A069088.
%K A074786 nonn
%O A074786 1,2
%A A074786 _Benoit Cloitre_, Sep 07 2002
%E A074786 More terms from _Michel Marcus_, Aug 09 2013