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A Symmetric TM is a TM which has a configuration graph that is undirected. That is configuration i yields configuration j if and only if j yields i. The set of languages that have a symmetric TM deciding it in log space is called SL. It was first defined in 1982 by Lewis and Papadimitriou,[1][2] who were looking for a class in which to place USTCON, which until this time could, at best, be placed only in NL, despite seeming not to require nondeterminism. They defined the symmetric Turing machine, used it to define SL. Symmetric Turing machines are kind of Turing machines with limited nondeterministic power. Informally Symmetric computations are reversible in an approximate manner. A symmetric computation is divided into locally deterministic sub computation (segments) between special configuration, where nondeterminism takes place, such that the only special configuration reached from a backward computation is the special configuration started the segment. Thus the segments are locally deterministic in its forward direction and have limited nondeterminism in its backward direction. Also if there is a way from special configuration A1 to special configuration A2, there must be a way back from A2 to A1.
Example of Symmetric Configuration
The most important Complete Problem is the USTCON, Lewis and Papadimitriou by their definition showed that USTCON is complete for SL class. They constructed a nondeterministic machine for USTCON, and they made a lemma for converting this machine into Symmetric Turing Machine. Then the theorem follows as any language can be accepted using a symmetric Turing machine is logspace reducible to USTCON as from the properties of the symmetric computation we can view the special configuration as the undirected edges of the graph.
