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I am trying to describe the edges of an undirected graph that contains loops. On Wikipedia they are characterized as 2-multisets, meaning it has two elements which can be identical, and the order is not important. I never heard of that term, and find no reference to it anywhere else, so I am not sure whether it is OK to use it in my thesis. Can anyone clear that up for me?

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    @IlmariKaronen : I think the set of edges would still be a set, not a multiset, with more than one edge between two vertices. If, for example, one were to assign weights to edges, as is sometimes done, then two edges between a pair of vertices might have different weights. But certainly each edge would still be a "2-multiset".2012-06-17

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Yes. See the Wikipedia article on multisets and the many references therein for where it has occurred previously in literature.

(Note, in the context, a 2-multiset merely means a multiset containing 2 elements.)

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As Willie Wong notes, it's a perfectly valid term, but at least I personally would find "unordered pair" more natural than "2-multiset".