A multiset is a set-like object in which order is ignored, but multiplicity is explicitly significant. Therefore, multisets 
and 
are equivalent, but 
and 
differ. The number of multisets of length 
on 
symbols, for a positive integer 
and a nonnegative integer 
, is called 
multichoose 
,
denoted 
.
Multiset
See also
Aggregate, Ball Picking, Binomial Coefficient, Choose, Collection, Combination, List, Multichoose, Multinomial Coefficient, Permutation, Set, StringExplore with Wolfram|Alpha
References
Skiena, S. Implementing Discrete Mathematics: Combinatorics and Graph Theory with Mathematica. Reading, MA: Addison-Wesley, p. 12, 1990.Referenced on Wolfram|Alpha
MultisetCite this as:
Weisstein, Eric W. "Multiset." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/Multiset.html