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 A081339 #16 Jan 22 2017 13:34:37 %S A081339 1,3,7,9,10,11,19,20,21,23,25,26,27,30,31,33,34,40,43,47,49,52,57,58, %T A081339 59,60,63,65,67,68,69,70,71,74,75,77,78,79,80,81,82,83,85,90,93,99, %U A081339 102,103,104,106,107,110,116,120,121,122,127,129,131,133,136,139,140,141,145 %N A081339 Numbers n such that sigma(n^2) modulo 4 = 1. %C A081339 Numbers n such that the sum of exponents of primes == 1 (mod 4) in the prime factorization of n is not congruent to n mod 2. - _Robert Israel_, Jan 22 2017 %H A081339 Robert Israel, <a href="/A081339/b081339.txt">Table of n, a(n) for n = 1..10000</a> %p A081339 filter:= proc(n) local F, t; %p A081339 F:= select(t -> t[1] mod 4 = 1, ifactors(n)[2]); %p A081339 (add(t[2],t=F) - n) mod 2 = 1; %p A081339 end proc: %p A081339 select(filter, [$1..1000]); # _Robert Israel_, Jan 22 2017 %t A081339 Select[Range[150],Mod[DivisorSigma[1,#^2],4]==1&] (* _Harvey P. Dale_, Apr 07 2012 *) %o A081339 (PARI) isok(n) = (sigma(n^2) % 4) == 1; \\ _Michel Marcus_, Jan 22 2017 %Y A081339 Cf. A000203, A065764, A081325. %Y A081339 Contains A004614. %K A081339 nonn %O A081339 1,2 %A A081339 _Benoit Cloitre_, Apr 20 2003