Unit sphere arcwise connected?

In summary, to show that the unit sphere in R^3 is arcwise connected, we need to find a continuous map between any two points on the sphere, such that the map stays on the sphere for all values of t between 0 and 1. One way to do this is by projecting a straight line path between the points onto the sphere. This may require dividing the vector by its length to ensure it stays on the sphere. This approach can also be used to show that the set is arc-connected, meaning any two points can be joined by a continuous curve on the sphere.
  • #1
chy1013m1
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Homework Statement


Show that the unit sphere {(x, y, z) : x^2 + y^2 + z^2 = 1} in R^3 is arcwise connected.


Homework Equations





The Attempt at a Solution


find a continuous map f(t ) such that f( 0) = a, f(1 ) = b. a, b, in R^3 and are on the unit sphere. then show for every t in [0,1] f(t ) is on the sphere. just having trouble finding the right f . Tried f = ta + (1-t)b but only noticed that it is a line segment , so it should be wrong..
 
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  • #2
a sphere is convex? I know balls are convex.. just by intuition, a line from a -> b (a, b, on the sphere, the 'shell') is not on the shell of the sphere. .? since {(x, y, z) : x^2 + y^2 + z^2 = 1} , equality is required not <= (i may be wrong)
 
  • #3
Just pick any path between the points that doesn't go through zero (eg, the straight line path unless they are diamterically opposed) and project it on the sphere by dividing each vector r(t) by its length. There is a slight technical difference between paths and arcs: arcs can't intersect themselves. But it shouldn't be hard to show this isn't a problem for the projections here.
 
  • #4
would all the points on the vector ab (a,b on the sphere) be on the sphere?.. i cannot see that and I don't think so..
I am just following the def of arc-connectness:
A set S in Rn is arc-connected if any 2 points in S can be joined by a continuous curve in S, that is, if for any a, b, in S, there is a continuous map f : [0,1] ->Rn s.t. f( 0) = a, f( 1) = b and f(t ) is in S for all t in [0, 1]
 
  • #5
No, they're not on the sphere. But StatusX told you how to get round that.
 

1. What is a unit sphere arcwise connected?

A unit sphere arcwise connected is a mathematical term that describes a set of points on a sphere that can be connected by a continuous curve. The curve must lie entirely on the surface of the sphere and cannot cross itself. This property is important in topology and geometry, as it helps to define the shape and structure of the sphere.

2. How is arcwise connectedness different from simple connectedness?

Simple connectedness refers to a space where every loop can be continuously shrunk to a single point. In contrast, arcwise connectedness only requires that two points can be connected by a continuous curve. A space can be arcwise connected but not simply connected, as seen in a torus shape.

3. What is the significance of unit sphere arcwise connectedness?

Unit sphere arcwise connectedness is significant because it helps to define the properties of a sphere and other geometric shapes. It also has applications in fields such as topology, where the connectedness of a space can affect its properties and behaviors.

4. Can a unit sphere be arcwise connected if it is not simply connected?

Yes, a unit sphere can be arcwise connected even if it is not simply connected. As mentioned before, a torus shape is an example of a space that is arcwise connected but not simply connected.

5. How is arcwise connectedness related to the concept of path-connectedness?

Arcwise connectedness is a stronger condition than path-connectedness. While path-connectedness only requires that two points can be connected by a continuous path, arcwise connectedness requires that the path must lie entirely on the surface of the sphere. Therefore, all spaces that are arcwise connected are also path-connected, but the reverse is not always true.

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