  Using the formalism of the Khalatnikov potential, we derive exact general
formulae for the entropy flow dS/dy, where y is the rapidity, as a function of
temperature for the (1+1) relativistic hydrodynamics of a perfect fluid. We
study in particular flows dominated by a sufficiently long hydrodynamic
evolution, and provide an explicit analytical solution for dS/dy. We discuss
the theoretical implications of our general formulae and some phenomenological
applications for heavy-ion collisions.
