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 A204826 #15 Feb 16 2025 08:33:16 %S A204826 1,2,4,8,9,16,25,32,49,50,64,81,98,121,128,169,225,242,256,289,338, %T A204826 361,441,484,512,529,578,625,676,722,729,841,961,1024,1058,1089,1156, %U A204826 1225,1250,1369,1444,1521,1681,1682,1849,1922,2025,2048,2116,2209,2312,2401 %N A204826 Deficient numbers with odd sum of divisors. %C A204826 Numbers m from A005100 such that sigma(m) = A000203(m) is odd. %C A204826 Complement of A204827 with respect to A005100 (deficient numbers). %H A204826 Amiram Eldar, <a href="/A204826/b204826.txt">Table of n, a(n) for n = 1..10000</a> %H A204826 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/DeficientNumber.html">Deficient Number</a> %e A204826 Deficient number 16 is in sequence because sigma(16) = 13 (odd number). %t A204826 aQ[n_] := OddQ[(s = DivisorSigma[1, n])] && s < 2n; Select[Range[2401], aQ] (* _Amiram Eldar_, Sep 02 2019 *) %Y A204826 Cf. A000203, A005100. %Y A204826 Cf. A156903 (abundant numbers with odd sum of divisors), A204825 (abundant numbers with even sum of divisors), A204827 (deficient numbers with even sum of divisors). %K A204826 nonn %O A204826 1,2 %A A204826 _Jaroslav Krizek_, Jan 22 2012