  The effect of the external field on the weakly-discontinuous first-order
phase transition is analyzed in the frame of the Landau theory. The
transformation of the free energy expansion as a power series in the order
parameter is suggested that maps the first-order phase transition onto the
second-order one under the 'effective' external field, that depends on both
temperature and on real field value. The presence of the third-degree term in
the Ginzburg-Landau expansion is shown to preserve the weakly-discontinuous
phase transition for some values of the external field in contrast with the
second-order phase transition. The case of proper ferroelastic (martensitic)
phase transition from cubic lattice to tetragonal one is considered and the
dependence of the transition temperature on the external hydrostatic as well as
uniaxial pressure is found.
