Search: id:A000076
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%I A000076 M1079 N0409
%S A000076 0,0,1,2,4,7,14,24,43,82,149,284,534,1015,1937,3713,7136,13759,26597,
%T A000076 51537,100045,194586,378987,739161,1443465,2821923,5522689
%N A000076 Number of integers <= 2^n of form 4 x^2 + 4 x y + 5 y^2.
%D A000076 D. Shanks and L. P. Schmid, Variations on a theorem of Landau. Part I, 
               Math. Comp., 20 (1966), 551-569.
%D A000076 N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 
               (includes this sequence).
%D A000076 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%H A000076 <a href="Sindx_Qua.html#quadpop">Index entries for sequences related 
               to populations of quadratic forms</a>
%e A000076 a(4)=4 since 2^4=16 and 4=4*1^2, 5=5*1^2, 13=4*1^2+4*1*1+5*1^2, 16=4*2^2.
%o A000076 (PARI) a(n)=if(n<0,0,sum(k=1,2^n,0<sum(j=0,sqrtint(k)\2,issquare(k-4*j^2)&(k-j)%2==0)))
%Y A000076 Sequence in context: A000075 A048248 A056180 this_sequence A054169 A065491 
               A072810
%Y A000076 Adjacent sequences: A000073 A000074 A000075 this_sequence A000077 A000078 
               A000079
%K A000076 nonn
%O A000076 0,4
%A A000076 N. J. A. Sloane (njas(AT)research.att.com).

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