  The formation of patterns of peaks on the free surface of a ferrofluid
subject to a magnetic field normal to the undisturbed interface is investigated
theoretically. The relative stability of ridge, square, and hexagon planforms
is studied using a perturbative energy minimization procedure. Extending
previous studies the finite depth of the fluid layer is taken into account.
Moreover, adding the wavenumber modulus $k$ to the set of variational
parameters also the wavenumber selection problem is addressed. The results are
compared with previous investigations and recent experimental findings.
