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The set $A= [z \in \mathbb C : |z+2|<2$ or $|z-2|<2]$ is not path connected.

I supposed that $A$ is path connected. Then there is a path , a continuous function $\gamma (t)=f_1(t)+if_2(t)$.

After that assumption how could I arrive at a contradiction ?

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    $\begingroup$ Your set $A$ is the union of two open balls with radius $2$, one has center $2$, the other has center $-2$. If $\gamma$ is a path in $A$, what can you say about the preimages of the open balls? $\endgroup$ Oct 23, 2015 at 17:37
  • $\begingroup$ yes, any further explanation please $\endgroup$ Oct 23, 2015 at 17:38
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    $\begingroup$ Well, what can you say about the preimages of these disjoint open balls in the interval $\Bbb I$, the domain of $\gamma$. How does this relate to the connectedness of $\Bbb I$? $\endgroup$ Oct 23, 2015 at 17:40
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    $\begingroup$ Well, $\Bbb I$ would then be a union of disjoint non-empty open sets if $\gamma$ meets both balls. But the interval is connected, so it cannot be written as such a union. $\endgroup$ Oct 23, 2015 at 17:44
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    $\begingroup$ Right, so there is no path connecting a point in one ball with a point in the other ball. Note that this is basically the same argument as in the proof that a path connected set is connected. $A$ is not connected. $\endgroup$ Oct 23, 2015 at 17:49

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First, notice that $B(-2,2)$ and $B(2,2)$ are convex and so path connected, so to show that $A$ is not path connected, you must show that there is no path between the two sets.

Suppose $\gamma$ is such a path, that is $\gamma(0) \in B(-2,2), \gamma(1) \in B(2,2)$.

Look at $r(t)=\operatorname{re} \gamma(t)$. Note that $r(0) <0, r(1) >1$ so there must be some $t$ such that $r(t) = 0$. However, the inaginary axis does not intersect $A$ hence a contradiction.

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Intersecting the balls of radius slightly greater than the balls you have with your subspace implies that your subspace isn't connected (since you will have two disjoint open non-trivial sets which cover the space).

Now, not connected implies not path-connected.

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