  Time evolutions whose infinitesimal generator is a fractional time derivative
arise generally in the long time limit. Such fractional time evolutions are
considered here for random walks. An exact relationship is given between the
fractional master equation and a separable continuous time random walk of the
Montroll-Weiss type. The waiting time density can be expressed using a
generalized Mittag-Leffler function. The first moment of the waiting density
does not exist.
