  Rational weak mixing is a measure theoretic version of Krickeberg's strong
ratio mixing property for infinite measure preserving transformations. It
requires "{\tt density}" ratio convergence for every pair of measurable sets in
a dense hereditary ring. Rational weak mixing implies weak rational ergodicity
and (spectral) weak mixing. It is enjoyed for example by Markov shifts with
Orey's strong ratio limit property. The power, subsequence version of the
property is generic.
