Skeleton of a category
A minimal full subcategory of a category that is equivalent to the category itself. In general, a category 
contains many skeletons. Any skeleton can be built up as follows. One chooses a representative in every isomorphism class of objects of 
. Then the full subcategory of 
generated by these objects is a skeleton of 
.
Two categories are equivalent if and only if their skeletons are isomorphic. A skeleton of a category inherits many properties of the category itself: local smallness, existence of a bicategory structure, various forms of completeness, etc.
Comments
A category is said to be skeletal if it is a skeleton of itself, that is, if no two distinct objects are isomorphic. A skeleton of 
may also be defined as a skeletal full subcategory which meets every isomorphism class of objects of 
. The construction of skeletons described above makes an obvious use of the axiom of choice; indeed, it can be shown that the assertions "every category has a skeleton" and "any two skeletons of a given category are isomorphic" are both equivalent to the axiom of choice, [a1].
References
| [a1] | P.J. Freyd, A. Scedrov, "Categories, allegories" , North-Holland (1990) |
Skeleton of a category. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Skeleton_of_a_category&oldid=33321