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 A243172 #32 Apr 09 2023 02:27:04
%S A243172 0,1,4,7,9,15,16,25,28,36,37,43,49,51,60,63,64,67,79,81,85,100,105,
%T A243172 109,112,121,123,127,135,141,144,148,151,163,169,172,175,177,193,196,
%U A243172 204,205,211,225,235,240,249,252,256,259,267,268,277,289,295,301,303,316,324,331,333,337,340,343,357,361,373,375,379,387,393,400,415,420,421,436,441,445,448,457,459,463,469,484,487,492,499
%N A243172 Nonnegative integers of the form x^2 + 3*x*y - 3*y^2 (discriminant 21).
%C A243172 Also numbers representable as x^2 + 5*x*y + y^2 with 0 <= x <= y. - _Gheorghe Coserea_, Jul 29 2018
%C A243172 Also numbers of the form x^2 - x*y - 5*y^2 with 0 <= x <= y (or x^2 + x*y - 5*y^2 with x, y nonnegative). - _Jianing Song_, Jul 31 2018
%C A243172 Also nonnegative numbers of the form 7*x^2 - 3*y^2. - _Jon E. Schoenfield_, Jun 03 2022
%H A243172 Gheorghe Coserea, <a href="/A243172/b243172.txt">Table of n, a(n) for n = 1..100001</a>
%H A243172 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 A243172 A={}; For[ n=0, n <= 300, n++,
%t A243172 If[ Length[ Reduce[x^2 + 3 x y - 3 y^2 - n == 0, {x,y}, Integers]]>0, AppendTo[A,n]]]; A
%o A243172 (PARI)
%o A243172 \\ From Bill Allombert, Jun 04 2014. Since 21 is a fundamental discriminant, and the polynomial is unitary, the following code works:
%o A243172 B=bnfinit(x^2+3*x-3); select(n->#bnfisintnorm(B,n),[1..500])
%Y A243172 Cf. A031363.
%Y A243172 Primes: A141159.
%K A243172 nonn
%O A243172 1,3
%A A243172 _N. J. A. Sloane_, Jun 01 2014