The minimum excluded value. The mex of a set 
of nonnegative
integers is the least nonnegative integer not
in the set.
Mex
See also
Mex SequenceExplore with Wolfram|Alpha
References
Guy, R. K. "Max and Mex Sequences." §E27 in Unsolved Problems in Number Theory, 2nd ed. New York: Springer-Verlag, pp. 227-228, 1994.Referenced on Wolfram|Alpha
MexCite this as:
Weisstein, Eric W. "Mex." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Mex.html