# Pathwise connected

Give an example: Let $A$ and $B$ be pathwise-connected subsets of $\mathbb{R}^2$, whose intersection $A?B$ is nonempty. The union $A?B$ is pathwise connected.

*Use the fact that a convex subset is pathwise connected for one of the subsets.

Example:
$A=\{(x,y)\in \mathbb{R}^2 | \hspace{1mm} |x|+|y| \leq 10\}$
$B=\{(x,y)\in \mathbb{R}^2 | \hspace{1mm} x^2+y^2=25\}$
and let $C=A \cup B$. Define the function $f:C\rightarrow \mathbb{R}$ by
$f(x,y)=\frac{5x+2y}{{\sqrt{x^2+y^2}}+1}$

Something similar to this example. But I need different subsets and function from this.

• Mathe

Are you sure you wrote the question correctly?

• Imanonymous

Which part is wrong?

• Mathe

Well, you could take A = B = R^2. All sets are convex and path-connected, A intersection B = R^2 is non empty and A union B = R^2 which is again convex and path-connected. One could also find easy examples with A different to B.

• Imanonymous

I have edited the question to make it clearer.

• Mathe

Please fix: " Let A and B of be pathwise-connected of \mathbb{R}^2". That statement is not correct.

• Imanonymous

Sorry for the mistake.

• Daniel90

The bounty is low.

• Nirenberg

You just need an example or you also need a proof that it satisfied the conditions?

• Imanonymous

A simple proof to show that each two subsets are pathwise connected or just hint would be fine.

• Imanonymous

I mean each subsets

• Mathe

I'm confused now. Do you need to prove that, "If A and B are path connected with non-empty intersection, THEN their union is also path connected"? Or do you need an example of this behavior? If it's just the example, I can answer that very quickly.

• Imanonymous

Just the example with explanation.

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• Imanonymous

Sorry can I request for a more complicated subsets? Just like the example I have given. I wish to have something like x^2+y^2=25 (e.g. a shape)

• Mathe

You can take A= {(x,y)| y>=x^2}, this is everything above the parabola y = x^2 and it is path connected. Take B = {(x,y)| y<=1-x^2}, this is everything below the parabola y = 1- x^2 . The intersection is not empty and their union is path connected.

• Imanonymous

Or can you help me to verify if I can define the two subsets as: A=[-3,3]x[-4,4] which is a generalised triangle and B={(x,y) | x^2+y^2=4}.

• Imanonymous

May I have some steps to show that y>=x^2 by using the definition of pathwise connected?

• Imanonymous

I also need a function with AUB as its domain, as shown in the example :)

• Mathe

The function sin(x+y) is defined on AUB.

• Mathe

The definition of path connected means that every pair of points in the set can be joined by a path. In this case, a line between two points will join them.

• Imanonymous

Can I have a function that is more complicated?

• Mathe

Sin((5x^2-2y^3)/(1+^x4+y^6))

• Imanonymous

Thank you!

• Imanonymous

Is each of these subsets is pathwise-connected? A=[-3,3]x[-4,4] which is a generalised triangle and B={(x,y) | x^2+y^2=4}

• Mathe

A=[-3,3]x[-4,4] is just a square, but yeah, they are path-connected.

• Imanonymous