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Prove that all contractible spaces are simply connected.

It's simple to prove that the space is pathwise connected. But, how can I prove that the fundamental group is trivial?

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marked as duplicate by ronno, Grigory M, mau, Daniel Rust, John May 8 at 9:48

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The fundamental group is homotopy invariant.

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How can I prove that the fundamental group is trivial? –  Henfe Feb 25 '13 at 20:09
    
Do you know the definition of a contractible space and what homotopy invariance means? –  Adeel Feb 25 '13 at 20:13
    
Contractible spaces yes, I know. But, homotopy invariance not yet. I'm a student of math and I am studying algebraic topology. I starded to study it one month ago. –  Henfe Feb 25 '13 at 20:16
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Take a loop $f:[0,1] \to X$. Contractibility of $X$ is equivalent to every map $Y \to X$ being null-homotopic so in particular $f$ is null-homotopic. So the fundamental group consists of the single element which is the class of a constant map. –  Adeel Feb 25 '13 at 20:34
    
@Adeel Thank's! –  Nicole Feb 25 '13 at 20:38
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Any closed curve can be contracted like the space.

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