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 A144922 #16 Jul 22 2025 06:12:07 %S A144922 1,4,6,9,12,16,18,20,24,25,28,36,44,45,48,49,50,54,60,64,72,81,90,92, %T A144922 96,100,108,112,117,121,132,140,144,150,153,162,168,169,180,192,196, %U A144922 198,200,204,216,225,228,234,240,242,252,256,270,288,289,294,300 %N A144922 Numbers k such that k*sigma_2(k)/sigma_1(k) is an integer. %C A144922 Numbers k such that k*A001157(k)/A000203(k) is an integer. This sequence is connected closely with Ore divisor numbers (A001599) and RMS numbers (A140480). %C A144922 This sequence is infinite. E.g., all the numbers of the form 3*2^m, for m >= 1, are terms, since 3*2^m * sigma_2(3*2^m) / sigma_1(3*2^m) = 5 * 2^(m-1) * (2^(m+1)+1) is an integer. - _Amiram Eldar_, Dec 25 2024 %H A144922 Amiram Eldar, <a href="/A144922/b144922.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1000 from Harvey P. Dale) %t A144922 Select[Range[300],IntegerQ[(#*DivisorSigma[2,#])/DivisorSigma[1,#]]&] (* _Harvey P. Dale_, Oct 28 2018 *) %o A144922 (PARI) is(k) = my(f = factor(k)); !((k*sigma(f, 2)) % sigma(f)); \\ _Amiram Eldar_, Dec 25 2024 %Y A144922 Cf. A000203, A001157, A001599, A140480. %Y A144922 A020487 is a subsequence. %K A144922 easy,nonn %O A144922 1,2 %A A144922 _Ctibor O. Zizka_, Sep 25 2008 %E A144922 More terms from _Harvey P. Dale_, Oct 28 2018