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%I #11 Feb 22 2020 20:06:00
%S 1,0,2,0,0,0,0,4,2,0,0,4,0,0,0,0,0,0,2,4,0,0,0,0,0,0,2,0,4,0,0,4,2,0,
%T 4,0,0,0,0,0,0,0,0,0,4,0,0,4,0,0,2,0,0,0,0,0,0,0,4,4,0,0,0,4,0,0,0,4,
%U 0,0,0,4,2,0,0,0,4,0,0,0,0
%N Expansion of (theta_3(z)*theta_3(13z) + theta_2(z)*theta_2(13z)).
%H Jinyuan Wang, <a href="/A028617/b028617.txt">Table of n, a(n) for n = 0..1000</a>
%t r[n_]:=Reduce[{x, y}.{{2, 1}, {1, 7}}.{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 (PARI) a(n) = if(n<1, n==0, qfrep([2, 1; 1, 7], n)[n]*2); \\ _Jinyuan Wang_, Feb 21 2020
%Y Cf. A028618, A028619, A028620.
%K nonn
%O 0,3
%A _N. J. A. Sloane_
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