A note on multi-time equations in quantum mechanics

The main aim of this note is to show that the formalism of multi-time equations in quantum mechanics meant to represent a manifestly Lorentz-invariant theory suffers from at least three imperfections: (1) It does not cover those physical systems for which the Schr"odinger picture of quantum mechanics does not exist. (2) The integrabiliy condition which has been proposed in the formalism does not imply as necessary that for every partial Hamiltonian there exists a Schr"odinger equation. (3) The formalism does not generally allow for interactions even in a non-relativistic multi-particle quantum system.