Continuous topology homework

1. Sep 29, 2006

ak416

Let X and X' denote a single set in the two topologies T and T', respectively. Let i: X' -> X be the identity function.
a) Show that i is continuous <=> T' is finer than T.

Ok im able to show that for any set in T|X this set is in T'. This is done as follows: Assume i is continuous. For any open set X^U (in X), i^-1(X^U) = X^U is open in X'. Since X' is open in the space with T', X^U is open in that space (i.e. X^U is an element of T').
However, i dont see how this necessarily applies to every open set defined by T. Maybe i interpreted the question wrong but i dont think so, so please help if you can..

2. Sep 29, 2006

matt grime

You're probably just missing some quantifiers in your statements. Please try to write everyting clearly and in full, grammatically correct English. There are several issues with what you wrote. How can i be the identity map unless X and X' are the same set, for instance?

It is trivial that if T and T' are two topologies on a space X that the identity from (X,T') to (X,T) is continuous if and only if T' is finer (has more open sets). But this is not what you wrote (though it couldbe what you meant to write).

Last edited: Sep 29, 2006
3. Sep 29, 2006

ak416

Thats exactly how it is written in Munkres-Topology 2nd edition p.111 and ya i am a little curious as to how that could be an identity function.