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.
%I A028958 #16 Jul 08 2025 18:58:17 %S A028958 0,1,4,6,8,9,12,16,18,23,24,25,26,27,32,36,39,48,49,52,54,58,59,62,64, %T A028958 72,78,81,82,87,92,93,94,96,100,101,104,108,116,117,121,123,124,128, %U A028958 138,141,142,144,146,150,156,162,164,167,169,173,174,184,186,188,192 %N A028958 Numbers represented by quadratic form with Gram matrix [ 2, 1; 1, 12 ] (divided by 2). %C A028958 Nonnegative integers of the form x^2 + x*y + 6*y^2, discriminant -23. - _Ray Chandler_, Jul 12 2014 %C A028958 The Gram matrix is positive-definite, therefore, if w := (1 + sqrt(-23)) / 2, then |x + w*y|^2 = x^2 + x*y + 6*y^2 > 0 for all integers x and y except x = y = 0. - _Michael Somos_, Mar 28 2015 %C A028958 The theta function of the lattice with basis [1, w] is the g.f. of A028959, therefore, A028959(n) is positive if and only if n is in this sequence. - _Michael Somos_, Mar 28 2015 %H A028958 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) %Y A028958 For primes see A033217. Cf. A028929, A106867. %Y A028958 Cf. A028959. %K A028958 nonn %O A028958 1,3 %A A028958 _N. J. A. Sloane_ %E A028958 More terms from Larry Reeves (larryr(AT)acm.org), Mar 29 2000