The Isomorphism class reference article from the English Wikipedia on 24-Apr-2004
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Isomorphism class

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An isomorphism class is a collection of mathematical objects isomorphic with a certain mathematical object. A mathematical object usually consists of a set and some mathematical relations and operations defined over this set.

Isomorphism classes are often defined if the exact identity of the elements of the set is considered irrelevant, and the properties of the structure of the mathematical object are studied.

Examples of this are ordinals and graphs.