closure of sets closed under a finitary operation
Theorem and proof
Since the theorem involves two different concepts of closure — algebraic and topological — we must be careful how we phrase it.
Let be the -ary operation, and suppose that is closed under this operation, that is, . From the fact that the closure of a product is the product of the closures (http://planetmath.org/ProductTopology), we have
From the characterization of continuity in terms of closure (http://planetmath.org/TestingForContinuityViaClosureOperation), we have
From the assumption that , we have
Putting all this together gives
If is a subgroup of a topological group , then is closed under both the group operation and the operation of inversion, both of which are continuous, and therefore by the theorem is also closed under both operations. Thus the closure of a subgroup of a topological group is also a subgroup.
It similarly follows that the closure of a normal subgroup of a topological group is a normal subgroup. In this case there are additional unary operations to consider: the maps for each in the group. But these maps are all continuous, so the theorem again applies.
Note that it does not follow that the closure of a characteristic subgroup of a topological group is characteristic, because this would require applying the theorem to arbitrary automorphisms of the group, and these automorphisms need not be continuous.
Straightforward application of the theorem also shows that the closure of a subring of a topological ring is a subring. Considering also the unary operations for each in the ring, we see that the closure of a left ideal of a topological ring is a left ideal. Similarly, the closure of a right ideal of a topological ring is a right ideal.
We also see that the closure of a vector subspace of a topological vector space is a vector subspace. In this case the operations to consider are vector addition and for each scalar the unary operation .
As a final example, we look at convex sets. Let be a convex subset of a real (or complex) topological vector space. Convexity means that for every the set is closed under the binary operation . These binary operations are all continuous, so the theorem again applies, and we conclude that is convex.
|Title||closure of sets closed under a finitary operation|
|Date of creation||2013-03-22 17:03:29|
|Last modified on||2013-03-22 17:03:29|
|Last modified by||yark (2760)|