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%I #17 Mar 14 2021 04:22:02
%S 0,1,4,7,9,13,16,19,25,27,28,31,36,37,43,49,52,61,63,64,67,73,76,79,
%T 81,91,97,100,103,108,109,112,117,121,124,127,133,139,144,148,151,157,
%U 163,169,171,172,175,181,189,193,196,199,208,211,217,223,225,229,241,243,244,247,252,256,259,268,271,277,279,283,289,292,301,304,307,313,316,324,325
%N Numbers of the form x^2 + xy + 7y^2.
%C Discriminant -27.
%C From _Jianing Song_, Mar 13 2021: (Start)
%C Numbers in A003136 that are not congruent to 3 modulo 9.
%C Closed under multiplication.
%C For k > 0, k is a term if and only if: write k = 3^a * Product_{i=1..r} (p_i)^(a_i) * Product_{i=1..s} (q_i)^(b_i), p_i == 1 (mod 3), q_i == 2 (mod 3) are primes, then a != 1 and each b_i is even. (End)
%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 Select[Range[0, 350], Resolve@Exists[{x, y}, Reduce[# == (x^2 + x y + 7 y^2), {x, y}, Integers]] &] (* _Vincenzo Librandi_, Feb 11 2020 *)
%Y Primes: A002476.
%Y Cf. A003136.
%K nonn
%O 1,3
%A _N. J. A. Sloane_, Jun 02 2014
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