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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.

A081339 Numbers n such that sigma(n^2) modulo 4 = 1.

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%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