  Bennett's pebble game was introduced to obtain better time/space tradeoffs in
the simulation of standard Turing machines by reversible ones. So far only
upper bounds for the tradeoff based on the pebble game have been published.
Here we give a recursion for the time optimal solution of the pebble game given
a space bound. We analyze the recursion to obtain an explicit asymptotic
expression for the best time-space product.
