Concept

Tag system

Summary
In the theory of computation, a tag system is a deterministic model of computation published by Emil Leon Post in 1943 as a simple form of a Post canonical system. A tag system may also be viewed as an abstract machine, called a Post tag machine (not to be confused with Post–Turing machines)—briefly, a finite-state machine whose only tape is a FIFO queue of unbounded length, such that in each transition the machine reads the symbol at the head of the queue, deletes a constant number of symbols from the head, and appends to the tail a symbol-string that depends solely on the first symbol read in this transition. Because all of the indicated operations are performed in a single transition, a tag machine strictly has only one state. Definitions A tag system is a triplet (m, A, P), where
  • m is a positive integer, called the deletion number.
  • A is a finite alphabet of symbols, one of which can be a special halting symbol. All finite (possibly empty) strings on A are calle
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