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 A028601 #10 Feb 22 2020 20:05:15
%S A028601 1,0,2,0,0,4,0,0,2,4,0,0,0,0,0,0,0,4,4,0,4,0,0,0,0,0,4,0,0,4,0,0,2,0,
%T A028601 0,0,4,0,0,0,0,4,0,0,0,8,0,0,0,0,6,0,0,4,0,0,0,0,0,0,0,0,0,0,0,8,0,0,
%U A028601 4,0,0,0,4,0,4,0,0,0,0,0,4,4,0,0,0,0,0,0
%N A028601 Expansion of (theta_3(z)*theta_3(9z) + theta_2(z)*theta_2(9z)).
%H A028601 Jinyuan Wang, <a href="/A028601/b028601.txt">Table of n, a(n) for n = 0..1000</a>
%t A028601 r[n_]:=Reduce[{x, y}.{{2, 1}, {1, 5}}.{x, y}==n, {x, y}, Integers]; Table[rn=r[n];Which[rn===False, 0, Head[rn]===Or, Length[rn], Head[rn]===And, 1], {n, 0, 105}] (* _Vincenzo Librandi_, Feb 22 2020 *)
%o A028601 (PARI) a(n) = if(n<1, n==0, qfrep([2, 1; 1, 5], n)[n]*2); \\ _Jinyuan Wang_, Feb 21 2020
%Y A028601 Cf. A028602, A028603, A028604.
%K A028601 nonn
%O A028601 0,3
%A A028601 _N. J. A. Sloane_
%E A028601 More terms from _Jinyuan Wang_, Feb 21 2020