RICHARD P. FEYNMAN ON TURING MACHINES, CA 1981

Autograph manuscript, 1 1/2 pp, in black ink, consisting of detailed notes written on:  TOFFOLI, TOMMASO. "Bicontinuous Extensions of Invertible Combinatorial Functions." Offprint from: Mathematical Systems Theory, pp 13-23, 14, 1981. (6 1/2 x 9 1/2 in). Stapled into original paper wrapper. First page inscribed by Toffoli "You may be interested in Section 2; those considerations must apply also to a Q.M. computer. T.T."

A GIANT OF PHYSICS ON A GIANT OF COMPUTING.  Feynman & Toffoli both attended the Physics of Computation Conference at MIT on May 6-8, 1981, and it is likely that Toffoli sent this offprint to Feynman in response to a conversation that the two had while there. Feynman evidently was interested in section 2 of the present offprint, as he sketches several diagrams in response to those found in the offprint, adding several annotations, and on the last page, further sketches out some detailed diagrams. Under the heading Turing he writes "CAN YOU MAKE REVERSIBLE TURING MACHINE. Machine state q. Read symbols. go to state q'.... GO FOR P STEPS (SAVE TAPE) KEEP RECORDS COPY TAPE REVERSE P STEPS RECORD...". A fantastic item, linking together two of the most celebrated minds of the 20th century.