  The classical Essam-Fisher and Rushbrooke relationships (1963) that connect
the equilibrium critical exponents of susceptibility, specific heat and order
parameter are shown to be valid only if the critical temperature is positive.
For quantum phase transitions (PT) with zero critical temperature, these
relations are proved to be of different form. This fact has been actually
observed experimentally, but the reasons were not quite clear. A general
formula containing the classical results as a special case is proposed. This
formula is applicable to all equilibrium PT of any space dimension. The
predictions of the theory are consistent with the available experimental data
and do not cast any doubts upon the scaling hypothesis.
