Entropy flow of a perfect fluid in (1+1) hydrodynamics

Using the formalism of the Khalatnikov potential, we derive exact general formulas 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 formulas and some phenomenological applications for heavy-ion collisions.