  We study the bottom of the spectrum of a magnetic hamiltonian with
axisymmetrical potential in R3. The associated magnetic field is planar,
unitary and non-constant. The problem reduces to a 1D-family of singular
Sturm-Liouville operators on the half-line. We study to associated band
functions and we compare it to the "de Gennes" operators arising in the study
of a 2D-hamiltonian with monodimensional, odd and discontinuous magnetic field.
We show in particular that the lowest energy is higher in dimension 3.
