For low rotation the frequency of a mode
of radial order n and degree l is splitted in 2l+1
components of azimuthal order 
and the splittings 
are given by:
where 
is
the unknown rotation rate versus depth r and colatitude 
(
) and Knlm(r,mu)
the so-called rotational kernels calculated for each mode from oscillation
eigenfunctions of an equilibrium solar model.
In order to investigate the rotation below 0.4R where the latitudinal dependence is particularly not well constrained because of the few azimuthal orders provided by the low l degree modes, one may want to simplify the problem and reduce it to 1D problem in radius. As a matter of fact 2D inversions are usually not able to peak averaging kernels in both radial and latitudinal direction below 0.4Rand two different 1D approximations of Equation 1 are used instead.
