Key idea:
V=Vx⊗Vy⊗Vz
a(q)=(a(q)0,…,a(q)Nq),q=0,…,mp=a(0)⊗⋯⊗a(m)pi0,i1,…,im=a(0)i1a(1)i1⋯a(m)im
ψp,q,r(x,y,z)=ˆψp(x)ˆψq(y)ˆψr(z)u(x,y,z)=∑p∈Ix∑q∈Iy∑r∈Izcp,q,rψp,q,r(x,y,z)