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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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Technically any term that you have well defined can be used in your thesis. – picakhu Jan 22 '11 at 18:25
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To illustrate what Picakhu said, see en.wikipedia.org/wiki/Boojum_%28superfluidity%29 and the Physics Today article by David Mermin linked therein. (Note: using a term and having it taken up by the community seriously are two different things.) – Willie Wong♦ Jan 22 '11 at 18:35
Or, for an older and more widely known term, see en.wikipedia.org/wiki/Quark#Etymology – Ilmari Karonen Jun 17 '12 at 15:59
This all seems to assume there can't be more than one edge between two vertices. – Michael Hardy Jun 17 '12 at 17:37
@MichaelHardy: Hmm? If there can be multiple edges between a pair of vertices, then the edges of the graph form a multiset rather than a set, but each edge is still an unordered pair (or a "2-multiset", if you prefer). – Ilmari Karonen Jun 17 '12 at 22:14
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2 Answers

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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".

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