  It is assumed that U atoms in $UGe_2$ have a number of $f$ electrons
appropriate to give them each a spin $s=1$ as well as one extra itinerant
electron which may equally well be on one or other U atom. The dynamical
degrees of freedom are spin-s operators of localized spins and spin-1/2 fermi
operators of itinerant electrons. Applying hydrostatic pressure changes the
bandwidths of spin-up and spin-down itinerant electrons in different way, which
leads to decreasing of the contribution of the fermions to the magnetization
keeping the spin-fermion interaction unchanged. In turn the local spin-fermion
interaction leads to ferromagnetic superconductivity. The model accounts, in a
quantitative and natural way, for the characteristics of the coexistence of
superconductivity and ferromagnetism in $UGe_2$, including many of the key
experimental results: metamagnetic transitions, quantum transition from
ferromagnetism to ferromagnetic superconductivity, the position of the highest
superconducting critical temperature etc.
