This paper describes the simulation of an S(n) spacebounded deterministic Turing machine by a reversible Turing machine operating in space S(n). It thus answers a question posed by Bennett in 1989 and refutes the conjecture, made by Li and Vitanyi in 1996, that any reversible simulation of an irreversible computation must obey Bennett’s reversible pebble game rules.

COMMUNICATIONSTO THE EDITOR 551 arteriovenous oxygen content difference (C[a-v]02) equals 3; and the fractional concentration of oxygen in the inspired gas (F1o2) equals 0.40. From the oxyhemoglobin dissociation curve, the arterial oxygen saturation equals 0.90. Since C(a-v)O, equals 3, the venous oxygen saturation (Sv02) equals 0.75 approximately. Using these figures, the QS/QT equals 0.502. T...