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 A069070 #30 Jul 29 2022 09:52:43 %S A069070 1,40,81,135,216,224,400,819,1372,3240,3744,4650,6318,18144,21700, %T A069070 27930,30240,32400,32760,69312,71148,91694,111132,174592,175500, %U A069070 185220,215472,241395,278318,293907,327600,336675,362700,386232,515450,958737 %N A069070 Numbers n such that n*sigma(n) is a perfect square. %C A069070 Also n such that the squarefree part of n (A007913) equals the squarefree part of sigma(n), A355928. %C A069070 Also n such that abundancy of n, sigma(n)/n is a rational square. - _Michel Marcus_, Oct 06 2013 %C A069070 See A230043, resp. A230538, for n whose abundancy is a rational cube, resp. fourth power. - _M. F. Hasler_, Nov 02 2013 %H A069070 Giovanni Resta, <a href="/A069070/b069070.txt">Table of n, a(n) for n = 1..1896</a> (terms < 4*10^12, first 500 terms from Donovan Johnson) %t A069070 Select[Range[1000000],IntegerQ[Sqrt[# DivisorSigma[1,#]]]&] (* _Harvey P. Dale_, Dec 24 2012 *) %o A069070 (PARI) for(n=1,1000000,if(issquare(n*sigma(n)),print1(n,","))) %o A069070 (PARI) isok(n) = issquare(sigma(n)/n); \\ _Michel Marcus_, Oct 06 2013 %Y A069070 Cf. A000203, A000290, A007913, A064987, A355928. %Y A069070 Cf. A008848, A027687 (subsequences). %Y A069070 Cf. also A230043, A230538. %Y A069070 Positions of 0's in A355929. %K A069070 easy,nonn %O A069070 1,2 %A A069070 _Benoit Cloitre_, Apr 05 2002 %E A069070 More terms from _Rick L. Shepherd_, Apr 07 2002