Is Any Closed Curve Homotopic to a Constant Curve in a Simply Connected Region?

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In summary, the definition of null homotopic states that a closed rectifiable curve in a region G is homotopic to zero if it is homotopic to a constant curve. If G is simply connected, then any curve gamma in G is homotopic to a constant curve, such as alpha(t)=3. However, this does not mean that gamma is homotopic to the constant curve beta(t)=0, as seen in the example of the open disk union D0 and D3. Therefore, it is important to note that even in a simply connected region, gamma may not be homotopic to "zero."
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I have a question about the definition of null homotopic. My textbook (Functions of one Complex Variable I by John B. Conway) defines it as follows: If [tex]\gamma[/tex] is a closed rectifiable curve in a region G, then [tex]\gamma[/tex] is homotopic to zero if [tex]\gamma[/tex] is homotopic to a constant curve. My question is, if G is simply connected, then is [tex]\gamma[/tex] homotopic to ANY constant curve in G? This seems obvious to me, but I'm not sure if I should prove it or just state it.
 
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Here's my joke: When is a curve that is homotopic to zero not homotopic to zero?

Answer: Let Dn be the open disk of radius 1 centered at n, and let G = union of D0 and D3. Then G is simply connected but not connected.

Let gamma be a curve in D3. Then gamma is homotopic to zero because it is homotopic to the constant curve alpha(t)=3.

However, gamma is not homotopic to the constant curve beta(t)=0, so one might say that gamma is not homotopic to "zero."
 
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Your G would not be simply connected :/ It has to be path connected and therefore connected to be simply connected.
 
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variety said:
I have a question about the definition of null homotopic. My textbook (Functions of one Complex Variable I by John B. Conway) defines it as follows: If [tex]\gamma[/tex] is a closed rectifiable curve in a region G, then [tex]\gamma[/tex] is homotopic to zero if [tex]\gamma[/tex] is homotopic to a constant curve. My question is, if G is simply connected, then is [tex]\gamma[/tex] homotopic to ANY constant curve in G? This seems obvious to me, but I'm not sure if I should prove it or just state it.
If G~c and G~c', then you would need to have c~c'...
 

1. What is the definition of null homotopic?

The term null homotopic refers to a continuous function between two spaces that can be continuously deformed to a constant function. In other words, the function can be shrunk to a single point without breaking or tearing the space.

2. How is null homotopy related to homotopy equivalence?

A null homotopy is a special case of homotopy equivalence, where the two spaces being compared are homotopic to each other. However, in a null homotopy, one of the spaces is a single point, while in a homotopy equivalence, both spaces are non-trivial.

3. Can you provide an example of a null homotopic function?

One example of a null homotopic function is the constant function on a circle. The circle can be continuously deformed to a single point, making the function null homotopic.

4. How does the concept of null homotopy relate to algebraic topology?

In algebraic topology, null homotopy is used to study the fundamental group of a space. If a space is null homotopic, its fundamental group is trivial, meaning it only contains the identity element. This helps to classify and distinguish between different topological spaces.

5. What is the importance of understanding null homotopy in mathematics?

Null homotopy is a fundamental concept in algebraic topology and has many applications in mathematics, such as in the study of homotopy groups, homology, and cohomology. It also helps to classify and compare topological spaces, providing a powerful tool for understanding their properties and structures.

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