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 A066112 #24 Feb 23 2025 02:40:26
%S A066112 1,16,36,48,49,64,81,100,121,144,162,180,196,225,245,256,324,361,400,
%T A066112 432,441,484,500,529,576,605,625,648,676,729,784,841,900,931,980,1024,
%U A066112 1089,1156,1200,1225,1280,1296,1369,1444,1521,1600,1620,1681,1764,1805
%N A066112 Numbers k such that sigma_4(k)/sigma_2(k) is an integer but not a prime.
%H A066112 Amiram Eldar, <a href="/A066112/b066112.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1000 from Harry J. Smith)
%e A066112 The sequence includes squares, twice squares (such as 162 and 648), and other numbers (such as 48 and 180). The sigma_4/sigma_2 quotients usually have more than one distinct prime factor. Exception: sigma_4(48)/sigma_2(48) = 5732210/3410 = 1681 = 41^2.
%t A066112 Do[s=DivisorSigma[4, n]; z=DivisorSigma[2, n]; If[IntegerQ[s/z]&&!PrimeQ[s/z], Print[n]], {n, 1, 10000}]
%o A066112 (PARI) isok(k) = { my(f=sigma(k, 4)/sigma(k, 2)); !frac(f) && !isprime(f) } \\ _Harry J. Smith_, Feb 01 2010
%Y A066112 Cf. A001157, A001159, A046871, A066109, A066110, A066111.
%K A066112 nonn
%O A066112 1,2
%A A066112 _Labos Elemer_, Dec 06 2001
%E A066112 Edited by _Jon E. Schoenfield_, Dec 24 2016