A028601 Expansion of (theta_3(z)*theta_3(9z) + theta_2(z)*theta_2(9z)).

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, 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, 4, 0, 0, 0, 4, 0, 4, 0, 0, 0, 0, 0, 4, 4, 0, 0, 0, 0, 0, 0

Offset: 0

Author

N. J. A. Sloane

Keywords

• nonn

Links

• Jinyuan Wang, Table of n, a(n) for n = 0..1000

Crossrefs

Cf. A028602, A028603, A028604.

Programs

Mathematica

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 *)

PARI

a(n) = if(n<1, n==0, qfrep([2, 1; 1, 5], n)[n]*2); \\ Jinyuan Wang, Feb 21 2020

Extensions

More terms from Jinyuan Wang, Feb 21 2020