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 A137816 #20 Sep 26 2024 22:19:47 %S A137816 5491,8075,25317,27455,71383,72283,76131,104975,138575,193041,203167, %T A137816 295569,295947,298775,334951,356915,361415,400843,451535,492275, %U A137816 509575,572975,589475,595975,654493,683757,815975,862087,876627,919075,936729 %N A137816 Nonsquarefree "year numbers" (numbers n such that phi(n) = 2*phi(sigma(n)): A137815). %C A137816 This is the subsequence of nonsquarefree elements of A137815. See there for more comments and references. %C A137816 There are only 32 such numbers below 10^6, 145 below 10^7 and 785 below 10^8. %H A137816 M. F. Hasler, <a href="/A137816/b137816.txt">Table of n, a(n) for n=1,...,785</a>. %t A137816 Reap[For[n=1, n<10^6, n++, If[!SquareFreeQ[n] && EulerPhi[n] == 2*EulerPhi[ DivisorSigma[1, n]], Print[n]; Sow[n]]]][[2, 1]] (* _Jean-François Alcover_, Dec 12 2016 *) %o A137816 (PARI) for( n=1,10^6, !issquarefree(n) && eulerphi(n)==2*eulerphi(sigma(n)) && print1(n", ")) %Y A137816 Cf. A137815-A137819, A006872, A067704. %K A137816 nonn %O A137816 1,1 %A A137816 _R. K. Guy_, _R. J. Mathar_ and _M. F. Hasler_, Feb 11 2008