for understanding of this subject
I recommend that one has familiarity with two dimensional flows, Navier Stokes solutions, spherical harmonics, Curl, Gradients, Flux, Divergence, volume elements, vector math the Laplacian the Legendrian and slope fields, as well as
{Oiler's} theorem
here one method to arrive at your answer from Abbott and Van Doenhoff
x
e =1/Ux
^8.210* INT[U^
8.210] dx
Sheesh
Happy New Year Pprune
PA