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PROG
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(Magma)
prec := 80;
S := SymmetricMatrix([4, 2, 4, 0, -2, 4, 0, -2, 0, 4, 0, 0, -2, 0, 4, -2, -2, 0, 0, 0, 4, 0, 0, 0, 0, 0, -2, 4, 0, 0, 0, 0, 0, 0, -2, 4, 0, 0, 0, 0, 1, -1, 0,
0, 4, 0, 0, 0, 0, -1, 0, 0, 0, 0, 4, -1, -2, 1, 1, 0, 1, 0, -1, 0, 1, 4, 0, -1, 0, 0, 1, 0, 1, 0, 0, 1, 1, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, -1, -1, 0, 4]);
L := LatticeWithGram(S);
M := ThetaSeriesModularFormSpace(L);
B := Basis(M, prec);
T<q> := ThetaSeries(L, 52);
Coefficients(&+[Coefficients(T)[2*i-1] * B[i] : i in [1..25]]+Coefficients(T)[53]*B[26]); // Andy Huchala, May 04 2023
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