  We consider a two dimensional reflecting random walk on the nonnegative
integer quadrant. It is assumed that this reflecting random walk has skip free
transitions. We are concerned with its time reversed process assuming that the
stationary distribution exists. In general, the time reversed process may not
be a reflecting random walk. In this paper, we derive necessary and sufficient
conditions for the time reversed process also to be a reflecting random walk.
These conditions are different from but closely related to the product form of
the stationary distribution.

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