magma://Please install CHIMP (https://github.com/edgarcosta/CHIMP) if you want to run this code
chi := DirichletCharacter("1998.73");
S:= CuspForms(chi, 2);
N := Newforms(S);
sage:from sage.modular.dirichlet import DirichletCharacter
H = DirichletGroup(1998, base_ring=CyclotomicField(6))
chi = DirichletCharacter(H, H._module([4, 3]))
N = Newforms(chi, 2, names="a")
This newform subspace can be constructed as the kernel of the linear operator
\( T_{5}^{72} - 116 T_{5}^{70} + 7428 T_{5}^{68} - 328096 T_{5}^{66} + 11036840 T_{5}^{64} + \cdots + 34828517376 \)
acting on \(S_{2}^{\mathrm{new}}(1998, [\chi])\).