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The number of elements in a Group 
, denoted 
.  The order of an element 
 of a finite group 
 is the
smallest Power of 
 such that 
, where 
 is the Identity Element.  In general, finding the order of
the element of a group is at least as hard as factoring (Meijer 1996).  However, the problem becomes significantly
easier if 
 and the factorization of 
 are known.  Under these circumstances, efficient
Algorithms are known (Cohen 1993).
See also Abelian Group, Finite Group
References
Cohen, H.  A Course in Computational Algebraic Number Theory.  New York: Springer-Verlag, 1993.
 
Meijer, A. R.  ``Groups, Factoring, and Cryptography.''  Math. Mag. 69, 103-109, 1996.