Invariance of domain

Invariance of domain is a theorem in topology about homeomorphic subsets of Euclidean space R n . It states: :If U is an between U and V . Normally, to check that f is a homeomorphism, one would have to verify that both f and its inverse function f  -1 are continuous; the theorem says that if the domain is an open subset of R n and the image is also in R n , then continuity of f  -1 is automatic. Furthermore, the theorem says that if two subsets U and V of R n are homeomorphic, and U is open, then V must be open as well. Both of these statements are not at all obvious and are not generally true if one leaves Euclidean space.

The theorem is due to L.E.J. Brouwer. Its proof uses tools of algebraic topology, notably the Brouwer fixed point theorem.

= Consequences =

An important consequence of the domain invariance theorem is that R n cannot be homeomorphic to R m if m ≠ n . Indeed, no non-empty open subset of R n can be homeomorphic to any open subset of R m in this case. (If n < m , then we can view R n as a subspace of R m , and the non-empty open subsets of R n are not open as subsets of R m .)

= Generalization =

The domain invariance theorem may be generalized to (meaning that f ( U ) is open in M whenever U is an open subset of M ).