M. Mattes et M. Sorg, INTEGRABILITY CONDITIONS, WAVE-FUNCTIONS, AND CONSERVATION-LAWS FOR THE RELATIVISTIC SCHRODINGER-EQUATIONS, Nuovo cimento della Societa italiana di fisica. B, Relativity, classical and statistical physics, 109(10), 1994, pp. 1097-1111
The theory of the relativistic Schrodinger equations is further elabor
ated: the integrability conditions for the existence of a wave functio
n psi(x) directly lead to the general situation where a wave function
(''pure state'') is not available to describe the physical system.