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A topological space is called simply connected if it is path-connected and every path between two points can be continuously transformed into any other such path while preserving the two endpoints in question.

I understand both formal and intuitive meaning of the simple connectedness, but I can't get the idea how they imply each other. For example, given the following 2D-picture of a space that is connected but not simply connected. I've tried to draw two paths that contradict to simple connectedness. I can't understand why we can't build a homeomorphism between them.

enter image description here

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    $\begingroup$ Can you show us such an homeomorphism ? What are the intermediate curves ? $\endgroup$
    – user65203
    Feb 3 '20 at 18:08
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    $\begingroup$ Homeomorphism is totally the wrong word here. The paths are, of course, homeomorphic, but they are NOT homotopic. $\endgroup$ Feb 3 '20 at 18:48
  • $\begingroup$ @TedShifrin thanks for the correction! $\endgroup$ Feb 3 '20 at 19:14
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First off, you are looking for a homotopy - not a homeomorphism - between the two paths.

Recall that a (path) homotopy between a path $\alpha: p_1 \rightsquigarrow p_2$ and a path $\beta: p_1 \rightsquigarrow p_2$ in a topological space $X$ is a continuous function $H: [0,1] \times [0,1] \rightarrow X$ s.t. $H(x,0) = \alpha(x)$ and $H(x,1) = \beta(x)$ and also $H(0,t) = p_1$ and $H(1,t) = p_2$.

One may also interpret this as a family of paths parametrized by $t \in [0,1]$ s.t. the starting point and the endpoint are the same for any of those paths, which varies continuously w.r.t. the parameter $t \in [0,1]$.

So in particular, the images of any of those paths needs to be contained in $X$. If you wanted to draw such a family of curves in your picture, they would either have to be discontinuous, of transition through the hole in the space. Either of which is a contradiction.

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  • $\begingroup$ Thanks a lot! I've understood! $\endgroup$ Feb 3 '20 at 19:12

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