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 A180097 #12 Sep 08 2019 02:51:58 %S A180097 1,3,7,21,22,30,31,46,51,55,66,70,71,81,85,93,94,102,107,110,115,119, %T A180097 127,138,142,154,156,159,165,170,187,199,210,213,214,217,230,235,238, %U A180097 253,255,265,282,291,310,318,321,322,330,343,345,355,357,364,371,374 %N A180097 Numbers n such that sigma(n) is powerful. %H A180097 Amiram Eldar, <a href="/A180097/b180097.txt">Table of n, a(n) for n = 1..10000</a> %e A180097 sigma(3)=2^2, sigma(7)=2^3, sigma(21)=2^5, sigma(66)=2^4*3^2. %p A180097 emin := proc(n::posint) local L; if n>1 then L:=ifactors(n)[2]; L:=map(z->z[2],L); min(L) else 0 fi end: L:=[]: for w to 1 do for n from 1 to 144 do sn:=numtheory[sigma](n); if emin(sn)>1 then L:=[op(L),n]; print(n,ifactor(n),sn,ifactor(sn)) fi; od; od; L; %t A180097 sigmaPowerQ[1] = True; sigmaPowerQ[n_] := Min@FactorInteger[DivisorSigma[1, n]][[;; , 2]] > 1; Select[Range[1000], sigmaPowerQ] (* _Amiram Eldar_, Sep 08 2019 *) %o A180097 (PARI) isok(n) = ispowerful(sigma(n)); \\ _Michel Marcus_, Sep 08 2019 %Y A180097 Cf. A001694, A000203, A180090, A180098. %K A180097 easy,nonn %O A180097 1,2 %A A180097 _Walter Kehowski_, Aug 10 2010 %E A180097 a(1) and more terms from _Amiram Eldar_, Sep 08 2019