# Prove A~B=>f(A)~f(B) for a continuous f:X->Y

• BiGyElLoWhAt

#### BiGyElLoWhAt

Gold Member
So proofs are a weak point of mine.
The hint is that a composite of a continuous function is continuous. I'm not really sure how to use that. What I was thinking was something to the effect of an epsilon delta proof, is that applicable?

Something to the effect of:
##A \sim B\text{ and let } f \text{ be a continuous function X} \to \text{Y:}##
##\text{by definition, if } f \text{ is continuous, then there exists an } \epsilon \text{ for every } \delta \text{ such that ... blah...so }##
##f(A) \sim f(A+\epsilon) \sim f(A+n\epsilon) \text{ and by induction } f(A) \sim f(B) \text{ for large enough n}##
is that strong enough? Since A+n*epsilon = B it goes to f(B). Would that be necessary in the proof?
How can I do a proof with composite continuity?

You will have to say what ##X## and ##Y## are, and what ##\sim## is.

At first I thought I was going to make an effort to understand the OP, but I just gave up.

At first I thought I was going to make an effort to understand the OP, but I just gave up.

My bet is that the OP means that if two numbers ##A## and ##B## are infinitesimally close, then ##f(A)## and ##f(B)## are also infinitesimally close. This is indeed a characterization of continuity in the hyperreal number system.

My bet is that the OP means that if two numbers A and B are infinitesimally close, then f(A) and f(B) are also infinitesimally close. This is indeed a characterization of continuity in the hyperreal number system
Both your bet and such characterisation sound plausible. I just experienced a parsing error when I encountered the "... blah...so".

Both your bet and such characterisation sound plausible. I just experienced a parsing error when I encountered the "... blah...so".

My parsing error came a bit earlier with "there is an epsilon for each delta"

My parsing error came a bit earlier with "there is an epsilon for each delta"
I remember a mischievous "true/false" question on an introductory analysis exam, where the definition of continuity was stated as usual, but with the roles of the symbols ##\varepsilon## and ##\delta## reversed, to the dismay of the audience.

I remember a mischievous "true/false" question on an introductory analysis exam, where the definition of continuity was stated as usual, but with the roles of the symbols ##\varepsilon## and ##\delta## reversed, to the dismay of the audience.

You mean the following?
$$\forall \delta >0: \exists \varepsilon>0: \forall x: |x-a|<\varepsilon~\Rightarrow~|f(x) - f(a)| < \delta$$
That's a really good question :D

You mean the following?
Yes, precisely. Could be worse:
A function ##x:\mathbb{R}\rightarrow \mathbb{R}## is continuous at ##\delta## if and only if
$$\forall a>0:~\exists \varepsilon>0: \forall f\in \mathbb{R}:~|f-\delta|<\varepsilon~\Rightarrow~|x(f)-x(\delta)|<a$$

• S.G. Janssens
Terribly sorry, all.
http://inperc.com/wiki/index.php?title=Homology_classes
There's where it comes from.
It isn't defined anywhere, but I'm assuming ~ means connected, no cuts/holes/ etc.
X is a subspace of R^n, and I'm assuming Y is as well, X is the only one that has actually been defined, but thus far, as has been given, we're only working in subspaces of R^n.

Does that clear things up? The Blah... is the usual definition for continuity in calculus, I was just being lazy.

*The exercise is about 1/3 of the way down, just before section 3 about counting features.

So ##A\sim B ## means that there is a continuous path ##q:[0,1]\rightarrow X## such that ##q(0)=A## and ##q(1)=B##? Can you find a continuous path between ##f(A)## and ##f(B)## then?

Yes, I believe.
I mean, intuitively, yes. The problem lies in the proof part, I think.
Am I supposed to use ##f(q): [0,1] \to Y## and then just the fact that q is continuous and f is continuous therefore f(q) is continuous?

What is ##f(q)## supposed to mean? Do you mean ##f\circ q##?

Yes, sorry. The composite.

I guess it would also be : [q(0), q(1)], would it not?

I guess it would also be : [q(0), q(1)], would it not?

That's an interval, that has no meaning in general topological spaces.

Yes, sorry. The composite.

Yes. And yes ##q## is continuous (by definition of being a path) and ##f## is continuous (given), so there composition ##f\circ q## is too.

• BiGyElLoWhAt
Is that literally it?

Do I not need to specify my interval that I'm mapping over to show continuity between two points?

Yes

• BiGyElLoWhAt
If you are not working on the Reals, you may not have an interval, period.

If you are not working on the Reals, you may not have an interval, period.

True, but in a vector space you usually have the following notation
$$[a,b] = \{ta+(1-t)b~\vert~t\in [0,1]\}$$
so perhaps he meant that?

Well if I'm looking between two points, and then looking between a map of those two points, wouldn't those be my 2 respective intervals?

Well if I'm looking between two points, and then looking between a map of those two points, wouldn't those be my 2 respective intervals?

Sorry, I'm not understanding this at all.

The idea was to show continuity between a 2 points in X after they have been mapped to Y. So wouldn't that be my interval?
##q : [A,B] \to f\circ q : [q(A),q(B)] ##
Or something, I'm not really good with the rigorous math notation. I hope you can interpret this as I think it would be.

True, but in a vector space you usually have the following notation
$$[a,b] = \{ta+(1-t)b~\vert~t\in [0,1]\}$$
so perhaps he meant that?
Sorry, I was referring to OP, trying to get him/her to clarify the assumptions.

EDIT: Besides, how do we know s/he is working in a topological vs?

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Well if I'm looking between two points, and then looking between a map of those two points, wouldn't those be my 2 respective intervals?
I don't know what you mean by continuity between two points. Would you clarify?

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EDIT: Besides, how do we know s/he is working in a topological vs?

In post 12, the OP specified ##X## and ##Y## to be subspaces of ##\mathbb{R}^n##.

• BiGyElLoWhAt
• BiGyElLoWhAt
By continuity I mean a continuous path betwwen points, so a continuous curve, or map (maybe I want to use the word transform here).

I'm not certain about the correct English wording, we call it path connected.
So A ~ B iff there is a path from A to B. As far as I understood it it's to show that f(A) ~ f(B) for continuous f. His / her intervals are the parametrization of the paths.
Exactly. Thank you for clarifying. You can use he, also, by the way.

Ah, I see, sorry. So you are asking whether continuity preserves path-connectedness? If that is the question, then the answer is then no; the topologist's sine curve is a counterexample.

If you are using the ε, δ definition of continuous functions, you will have to trace the ε, δs step by step through the composition of the functions. If you are using the open set definition (the preimage of every open set is open), then the proof is easier. Just say that for every open set, O, in the range of f(g), f-1(O) is open because f is continuous; and g-1(f-1(O)) is open because g is continuous, so f(g) is continuous.

The only thing remaining is to verify the end-point condition of the definition of "homologous"

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• BiGyElLoWhAt and fresh_42