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The unary numeral system is the simplest system for representing natural numbers. In using the system, an arbitrary symbol is chosen; in order to represent a number N, the chosen symbol is repeated N times. For example, if we choose the symbol X, the number 5 is represented as XXXXX. The number of symbols is thus in one-to-one correspondence with the number to be represented. Counting on one's fingers is effectively a unary system. Unary may also be considered the base-1 numeral system, with the exception that the unary digit represents the number 1 instead of 0, unlike numeral systems of other bases, where digits typically range from 0 to the base minus one.

Compared to positional numeral systems, the unary system is inconvenient and is not used in practice. It would be cumbersome, for instance, to calculate 500 + 700 in the unary system. It occurs in some problem descriptions in theoretical computer science (e.g. some P-Complete problems), where it is used to "artificially" decrease the run-time or space requirements of a problem. For instance, the problem of integer factorization is suspected to require more than polynomial run-time if the input is given in binary, but it only needs linear runtime if the input is presented in unary.

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