Log-space transducer

A log space transducer (LST) is a type of Turing machine used for log-space reductions.

A log space transducer has three tapes:

• A read-only input tape.
• A read/write work tape.
• A write-only, write-once output tape.

A language A is said to be log-space reducible to a language B if there exists a LST which will convert an input from problem A into an input to problem B, using logarithmic space on the work tape, such that the input is in A iff the output is in B.

This seems like a rather convoluted idea, but it has two useful properties that are desirable for a reduction:

1. The property of transitivity holds. (A reduces to B and B reduces to C implies A reduces to C).
2. If A reduces to B, and B is in L, then we know A is in L.

Transitivity holds because it is possible to feed the output tape of one reducer (A->B) to another (B->C). At first glance, this seems incorrect because the A->C reducer needs to store the output tape from the A->B reducer onto the work tape in order to feed it into the B->C reducer, but this is not true. Each time the B->C reducer needs to access its input tape, the A->C reducer can re-run the A->B reducer, and so the output of the A->B reducer never needs to be stored entirely at once.

References

• Szepietowski, Andrzej (1994), Turing Machines with Sublogarithmic Space , Springer Press, ISBN 3-540-58355-6. Retrieved on 2008-12-03.