result

Now express ψ(θ) in terms of η and φ derivatives. This gives eq.(3.8) of SH02.

psi[0] + (se[l1]*se[u1])/2 + 
 (o*se[l1]*se[l2]*se[l3]*tp3[u1, u2, u3])/3 + 
 (o^2*se[l1]*se[l2]*se[l3]*se[l4]*tp4[u1, u2, u3, u4])/8