Derivation of the wave equation
  We shall formulate the postulates, from which the wave equation can be obtained. In the end it will be understandable, that it is the most natural law describing the motion of a system. The postulates themselves are simple and apparent: the first and the fifth postulate set the definition of the coordinate and the definition of its evolution. The second and the fourth postulate define the momentum and define its evolution. The third postulate sets the superposition principle. On the basis of these statements it is possible to obtain the wave equation. 
Benjamin Titov