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 A248151 #22 Apr 03 2025 19:01:49 %S A248151 1,2,4,5,8,9,10,13,16,17,18,20,25,26,29,32,34,36,37,40,41,45,49,50,52, %T A248151 53,58,61,64,68,72,73,74,80,81,82,89,90,97,98,100,101,104,106,109,113, %U A248151 116,117,121,122,128,136,137,144,146,148,149,153,157,160,162,164,169,173,178,180 %N A248151 Numbers n such that the sum of the divisors of n is not divisible by 4. %C A248151 The complement of A248150; contains in particular A028982, the squares and twice the squares, for which sigma is odd. %C A248151 The terms cannot have more than one odd prime factor to an odd power. Therefore this set has asymptotic density zero. The sequence grows faster than .75 n log(n). In particular: a(10) = 17, a(100) = 298, a(10^3) = 4724, a(10^4) = 66533, a(10^5) = 872434, a(10^6) = 10819205. - _M. F. Hasler_, Apr 26 2017 %H A248151 M. F. Hasler, <a href="/A248151/b248151.txt">Table of n, a(n) for n = 1..1000</a> %t A248151 Select[Range[200],Mod[DivisorSigma[1,#],4]!=0&] (* _Harvey P. Dale_, Apr 03 2025 *) %o A248151 (PARI) for(n=1,999,sigma(n)%4&&print1(n",")) %Y A248151 Cf. A000203 (sum of divisors), A028982, A028983, A248150. %Y A248151 Subsequence of A285801. %K A248151 nonn %O A248151 1,2 %A A248151 _M. F. Hasler_, Oct 02 2014