# Complete set of invariants

In mathematics, a complete set of invariants for a classification problem is a collection of maps

(where *X* is the collection of objects being classified, up to some equivalence relation, and the are some sets), such that if and only if for all *i*. In words, such that two objects are equivalent if and only if all invariants are equal.^{[1]}

Symbolically, a complete set of invariants is a collection of maps such that

is injective.

As invariants are, by definition, equal on equivalent objects, equality of invariants is a *necessary* condition for equivalence; a *complete* set of invariants is a set such that equality of these is *sufficient* for equivalence. In the context of a group action, this may be stated as: invariants are functions of coinvariants (equivalence classes, orbits), and a complete set of invariants characterizes the coinvariants (is a set of defining equations for the coinvariants).

## ExamplesEdit

- In the classification of two-dimensional closed manifolds, Euler characteristic (or genus) and orientability are a complete set of invariants.
- Jordan normal form of a matrix is a complete invariant for matrices up to conjugation, but eigenvalues (with multiplicities) are not.

## Realizability of invariantsEdit

A complete set of invariants does not immediately yield a classification theorem: not all combinations of invariants may be realized. Symbolically, one must also determine the image of