When is the wigner quasiprobability density nonnegative?
Abstract
It is shown that a necessary and sufficient condition for the Wigner quasiprobability density to be a true density is that the corresponding Schrödinger state function be the exponential of a quadratic polynomial.
 Publication:

Reports on Mathematical Physics
 Pub Date:
 October 1974
 DOI:
 10.1016/00344877(74)90007X
 Bibcode:
 1974RpMP....6..249H