What is the definition and use of homotopy in curve mapping?

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In summary, a curve is a map from [0,1] to D with a parameter t, while a homotopy is a map from two intervals [0,1] to D with parameters t and s. The homotopy continuously deforms one curve to another by varying s from 0 to 1, creating a continuous curve for each fixed t.
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Woolyabyss
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I'm having difficulty understanding the definition of homotopy.
In the notes attached I'm not sure what the mapping does and why the cartesian product is used. does t represent the parameter and s any curve between phi(1) and phi(2)?
Any help would be appreciated
 

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A curve is a map from [0,1] to D, where the parameter in [0,1] can be called t. The homotopy on the other hand is a map from two intervals [0,1] to D, with the two parameters now being called t and s, such that for s=0, the remaining map from [0,1] is one of the curves and for s=1 it is the other. This is what is meant by continuously deforming one curve to the other, each point (fixed t) along the curve makes another continuous curve, parametrised by s, and for each fixed s you have some continuous curve.
 
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1. What is meant by "homotopy of two curves"?

Homotopy of two curves refers to the continuous deformation of one curve into another on a given surface while keeping its endpoints fixed. This concept is often used in topology to study the properties of curves and their relationship with each other.

2. How is the homotopy of two curves represented mathematically?

The homotopy of two curves can be represented mathematically using the concept of a homotopy function. This function takes two parameters, t and s, and maps them to the points on the curves, resulting in a continuous deformation from one curve to the other as t and s vary from 0 to 1.

3. What are the key properties of homotopy of two curves?

There are several important properties of homotopy of two curves, including the fact that it is an equivalence relation, meaning it is reflexive, symmetric, and transitive. It is also compatible with composition, meaning that if two curves are homotopic, then any compositions of those curves will also be homotopic.

4. How is homotopy of two curves used in real-world applications?

Homotopy of two curves has many applications in various fields, including physics, computer science, and engineering. It is used to study the properties of knots, which are important in fields such as biology and chemistry. It is also used in computer graphics to create smooth animations and in robotics to plan efficient paths for robots.

5. Can two curves be homotopic if they have different endpoints?

No, two curves cannot be homotopic if they have different endpoints. This is because the definition of homotopy requires the endpoints of the curves to remain fixed during the deformation. If the endpoints are different, then the curves cannot be continuously deformed into each other while keeping the endpoints fixed.

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