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we all know that a single capital letter (e.g., $S$) usually represent a set (containing non-duplicate objects) and we can write a number of operations on set such as $|S|$, $|S|$ union $|S'|$ etc.

However in my case I need to define a list, that is duplicate objects are allowed, and still want to be able to use $|\cdot |$ to denote the size of the list.

Is there such kind of math notations?

Many thanks!

flonk
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Ziqi
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    Welcome to math.SE! If not only the possibility of duplicate elements, but also the order of the elements plays a role, this data structure is called a list. Though it is a mathematical concept, it is more commonly used in computer sciences. – flonk Mar 05 '14 at 13:33
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    When duplicates are allowed, but order does not matter, it may be called a multiset. There is no standard notation for lists or multisets that everyone will understand, so before you use them, specify your own notation, then go ahead and use it. Don't just say $|S|$ is the size of $S$, but say what you mean by "size". – GEdgar Mar 05 '14 at 13:44
  • Does the order of the items matter? Is $\langle p,r,r\rangle$ different from $\langle r,p,r\rangle$? – MJD Mar 05 '14 at 15:20

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