Isomorphism class
From Wikipedia, the free encyclopedia
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. However, there are circumstances in which the isomorphism class of an object conceals vital internal information about it; for example, in homotopy theory, the fundamental group of a space X at a point p, though technically denoted π1(X,p) to emphasize the dependence on the base point, is often written lazily as simply π1(X) if X is path connected. The reason for this is that the existence of a path between two points allows one to identify loops at one with loops at the other; however, unless π1(X,p) is abelian this isomorphism is non-unique. Furthermore, the classification of covering spaces makes strict reference to particular subgroups of π1(X,p), specifically distinguishing between isomorphic but conjugate subgroups, and therefore amalgamating the elements of an isomorphism class into a single featureless object seriously decreases the level of detail provided by the theory.
