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A028649
Expansion of (theta_3(z)*theta_3(21z) + theta_2(z)*theta_2(21z)).
1
1, 0, 2, 0, 0, 0, 0, 0, 2, 0, 0, 4, 0, 0, 0, 4, 0, 0, 2, 0, 0, 0, 0, 4, 0, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 4, 0, 0, 0, 0, 0, 0, 2, 0, 4, 0, 0, 0, 0, 0, 6, 4, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 2, 0, 4, 0, 0, 0, 0, 0, 0
OFFSET
0,3
LINKS
MATHEMATICA
r[n_]:=Reduce[{x, y}.{{2, 1}, {1, 11}}.{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 23 2020 *)
PROG
(PARI) a(n) = if(n<1, n==0, qfrep([2, 1; 1, 11], n)[n]*2); \\ Jinyuan Wang, Feb 21 2020
CROSSREFS
KEYWORD
nonn
STATUS
approved