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Bacle2 is offline
Feb7-12, 09:43 AM
Sci Advisor
P: 1,168
Quote Quote by Jooolz View Post
Hi all,

I am struggling with the following:

If X and Y are topological spaces. and f: X x Y → ℝ is a continuous function (product topology on X x Y, Euclidean topology on ℝ)

Let g: X → ℝ defined by g(x) = sup { f(x,y) | y in Y }

Then: If A=(r, ∞) for r in ℝ, g-1(A) is open. And If A=(-∞, t) for t in ℝ, g-1(A) is not always open.

Why is that? How can I know if g-1(A) is open or not if I dont know anything about X??

Does anyone have an idea?

kind regards,

Well, let's see. Let me review for next time I teach point set topology, so you can
combine it with Zhentil's answer:

What is g-1(a,b)? it is the collection of all x such that there is a y in Y
with a<f(x,y)<b.

We have g=Sof(x,y) , where f:XxY→f(XxY), and S:f(XxY)→ℝ , and g-1:=


Like Zhentil said, if there is y0 with f(x,y0)>a , then , by

(assumed) continuity of f, there is a ball B(x,y0) where f(x,y)>a . This

gives you openness in the subspace f(XxY). Now, compose with f-1, to

get an open set in XxY, by assumed continuity of f(x,y).