How Can Compactness and the Tychonoff Theorem Simplify Functions in Topology?

[cogito²]

My question comes from homework from a section on the Tychonoff Theorem. This is the question:

Let Q = I^A be a cube, and let f be a continuous real-valued function on Q. Prove, given \epsilon > 0, there is a continuous real-valued function g on Q such that |f - g| < \epsilon and g is a function of only a finite number of coordinates. [Hint: Cover the range of f by a finite number of intervals of length \epsilon and look at the inverse images of these intervals.]

Now I have an idea about how to go about this. I know that Q is compact since I = [0,1] is and the Tychonoff Theorem states that the product of compact spaces is compact. I then know that f(Q) \subset \mathbb{R} must be compact, since f is continuous. I then know that it is closed and bounded meaning that there is an interval [-M,M] such that f(Q) \subset [-M,M]. I can then cut that interval into a finite number of (non-disjoint) open sets of length \epsilon.

I would then look at their inverse images and see that they are open sets covering Q. I know that those open sets are unions of base sets that are products of sets of which only a finite number are not equal to I (because this is the product topology). The problem for me is here is that the open sets cannot necessarilly be written as products (of sets) so I don't see how I can ignore all but a finite number of coordinates. What I would like is for the open sets to be a finite union of base sets. If that were true than I could find a finite number of coordinates that it belongs to but if not than I don't see how I can define it.

Beyond that I would like to define my function g as a sum of functions g_i each defined on the partitions of [-M,M]. I would be tempted to just define the functions to have a value of part of the partition but then that would result in a step function that wouldn't be continuous anyway. If I could come up with a set of continuous functions \{g_i\} then my plan is to use the Partition of Unity to combine them into one function x that would be the result of the problem.

So I'm pretty stumped at the moment. I just don't see how to simplify it. Hopefully my explanation is understandable. Any help would be greatly appreciated.

 

[wais]

Topology and compactness are important concepts in mathematics, particularly in the field of analysis. Topology deals with the study of the properties of spaces that are preserved under continuous transformations, while compactness is a property of a space that ensures that certain functions defined on that space have nice properties. In this question, we are dealing with the Tychonoff Theorem, which states that the product of compact spaces is compact. This theorem is a powerful tool in topology and is often used to prove various results in analysis.

To solve this problem, we need to use the fact that Q = I^A is compact, as I is compact and the product of compact spaces is compact. We also know that f is continuous on Q, which means that f(Q) \subset \mathbb{R} is compact, closed, and bounded. This allows us to cover f(Q) with a finite number of intervals of length \epsilon. Now, we need to look at the inverse images of these intervals and use the fact that they are open sets covering Q. This means that they can be written as unions of base sets, where only a finite number of these sets are not equal to I. This is because we are dealing with the product topology, where the open sets are defined as unions of products of sets.

However, the problem arises when we try to simplify these open sets into finite unions of base sets. This is because the open sets may not necessarily be written as products of sets, making it difficult to ignore all but a finite number of coordinates. To overcome this, we can use the Partition of Unity, which allows us to combine a finite number of continuous functions to create a new continuous function. By defining our function g as a sum of functions g_i, each defined on the partitions of [-M,M], we can use the Partition of Unity to combine them into one function that satisfies the conditions of the problem.

In summary, to solve this problem, we need to use the fact that Q is compact and f is continuous, and then use the properties of compactness and topology to cover f(Q) with a finite number of intervals. From there, we can use the Partition of Unity to combine a finite number of continuous functions to create a new function g that satisfies the conditions of the problem. This may seem like a complex process, but it is a common approach in topology and analysis problems. With practice, you will become more familiar with these concepts and be able to