%I A001826
%S A001826 1,1,1,1,2,1,1,1,2,2,1,1,2,1,2,1,2,2,1,2,2,1,1,1,3,2,2,1,2,2,1,1,2,2,2,
%T A001826 2,2,1,2,2,2,2,1,1,4,1,1,1,2,3,2,2,2,2,2,1,2,2,1,2,2,1,3,1,4,2,1,2,2,2,
%U A001826 1,2,2,2,3,1,2,2,1,2,3,2,1,2,4,1,2,1,2,4,2,1,2,1,2,1,2,2,3,3,2,2,1,2,4
%N A001826 Number of divisors of n of form 4k+1.
%H A001826 Nick Hobson, <a href="b001826.txt">Table of n, a(n) for n = 1..10000</
               a>
%H A001826 Michael Gilleland, <a href="selfsimilar.html">Some Self-Similar Integer 
               Sequences</a>
%F A001826 G.f.: Sum_{n>0} x^n/(1-x^(4n)) = Sum x^(4n+1)/(1-x^(4n+1)), n=0..inf.
%F A001826 a(n) = A001227(n) - A001842(n). - Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), 
               Apr 18 2006
%p A001826 d:=proc(r,m,n) local i,t1; t1:=0; for i from 1 to n do if n mod i = 0 
               and i-r mod m = 0 then t1:=t1+1; fi; od: t1; end; # no. of divisors 
               i of n with i == r mod m
%o A001826 (PARI) a(n)=if(n<1,0,sumdiv(n,d,d%4==1))
%Y A001826 Sequence in context: A046951 A159631 A050377 this_sequence A003641 A165190 
               A025890
%Y A001826 Adjacent sequences: A001823 A001824 A001825 this_sequence A001827 A001828 
               A001829
%K A001826 nonn
%O A001826 1,5
%A A001826 N. J. A. Sloane (njas(AT)research.att.com).
%E A001826 Better definition from Michael Somos, Apr 26 2004