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%I #20 Sep 08 2022 08:44:45
%S 0,1,4,7,8,9,11,16,23,25,28,29,32,36,37,43,44,49,53,56,63,64,67,71,72,
%T 77,79,81,88,92,99,100,107,109,112,113,116,121,127,128,137,144,148,
%U 149,151,161,163,169,172,175,176,179,184,191,193,196,197,200,203,207,211,212,224
%N Numbers of form x^2 + 7y^2.
%H Charles R Greathouse IV, <a href="/A020670/b020670.txt">Table of n, a(n) for n = 1..10000</a>
%H N. J. A. Sloane et al., <a href="https://oeis.org/wiki/Binary_Quadratic_Forms_and_OEIS">Binary Quadratic Forms and OEIS</a> (Index to related sequences, programs, references)
%t lim=250; k=7; Union@Flatten@Table[x^2 + k y^2, {y, 0, Sqrt[lim/k]}, {x, 0, Sqrt[lim-k y^2]}] (* _Vincenzo Librandi_, Aug 31 2016 *)
%o (PARI) is(n)=my(f=factor(n));for(i=1,#f[,1],if(kronecker(f[i,1],7)<0 && f[i,2]%2, return(0))); n%4!=2 \\ _Charles R Greathouse IV_, Nov 18 2012
%o (Magma) [n: n in [0..230] | NormEquation(7, n) eq true]; // _Vincenzo Librandi_, Aug 31 2016
%Y Cf. A033207.
%K easy,nonn
%O 1,3
%A _David W. Wilson_
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