. Each

Bennett's pebble game [1, 2] was introduced to obtain better time/space tradeos in the simulation of standard Turing machines by reversible ones. So far only upper bounds for the tradeo 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.