  We present a general thermodynamic theory that describes phases and phase
transitions of ferromagnetic superconductors with spin-triplet electron Cooper
pairing. The theory is based on extended Ginzburg-Landau expansion in powers of
superconducting and ferromagnetic order parameters. We propose a simple form
for the dependence of theory parameters on the pressure that allows correct
theoretical outline of the temperature-pressure phase diagram for which at low
temperatures a stable phase of coexistence of p-wave superconductivity and
itinerant ferromagnetism appears. We demonstrate that the theory is in an
agreement with the experimental data for some intermetallic compounds that are
experimentally proven to be itinerant ferromagnetic exhibiting spin-triplet
superconductivity. Some basic features of quantum phase transitions in such
systems are explained and clarified. We propose to group the spin-triplet
ferromagnetic superconductors in two different types of thermodynamic behavior,
on the basis of quantitative criterion deduced from the present theory and the
analysis of experimental data.
