# SageMath code for working with modular form 405.2.j.a


# Compute space of new eigenforms: 
from sage.modular.dirichlet import DirichletCharacter
H = DirichletGroup(405, base_ring=CyclotomicField(6))
chi = DirichletCharacter(H, H._module([2, 3]))
N = Newforms(chi, 2, names="a")

# select newform: 
traces = [4,-3,0,3,-6,0,-12,0,0,-1]
f = next(g for g in N if [g.coefficient(i+1).trace() for i in range(10)] == traces)

# q-expansion: 
f.q_expansion() # note that sage often uses an isomorphic number field