Prove intersection of convex cones is convex.

In summary: If one cone is a subset of the other, then their union will still be a convex cone, as all elements in the smaller cone will also be in the larger cone. In summary, we discussed the properties of convex cones and how they relate to the intersection and sum of two convex cones. We also touched on the question of whether the union of two convex cones is always a convex cone, and determined that it is not always the case, but it depends on the specific cones involved. It is important to show effort and understanding when asking for help with problems related to convex cones.
  • #1
jvt05
4
0
1. Let A and B be convex cones in a real vector space V. Show that A[itex]\bigcap[/itex]B and A + B are also convex cones.
 
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  • #2
First state the definition of a convex cone, and then try to prove it. People will help if you show some effort first.
 
  • #3
well set C is a convex cone if for any x,y [itex]\in[/itex] C and any scalars a≥0, b≥0, ax + by [itex]\in[/itex] C

so let A and B be convex cones.
A[itex]\bigcap[/itex]B would contain all elements x [itex]\in[/itex] both A and B.
This is where I am having trouble.
 
  • #4
other properties of convex cones:

1. for any positive scalar α and any x [itex]\in[/itex] C, the vector αx = (α/2)x + (α/2)x is in C.

2. set C is a convex cone if and only if αC = C and C + C = C.

perhaps my trouble is coming from the fact that I do not fully understand how these properties work.
 
  • #5
jvt05 said:
well set C is a convex cone if for any x,y [itex]\in[/itex] C and any scalars a≥0, b≥0, ax + by [itex]\in[/itex] C

so let A and B be convex cones.
A[itex]\bigcap[/itex]B would contain all elements x [itex]\in[/itex] both A and B.
This is where I am having trouble.

I'm having problems trying to follow your problem here. If x and y are in A then ax+by is in A, right? If x and y are in B then ax+by is in B, also right? If x and y are in AnB then they are in BOTH A and B. Doesn't that make ax+by in BOTH A and B? Hence an element of AnB?
 
  • #6
Hello. I don't want to create a new topic. I have very similar question about convex cone. I know what intersection and sum of two convex cones are also convex cone. But what's about union. The answer is that union of two convex cones may not be convex cone. But I can't understand why? Any thoughts? Thanks
 
  • #7
trenekas said:
Hello. I don't want to create a new topic. I have very similar question about convex cone. I know what intersection and sum of two convex cones are also convex cone. But what's about union. The answer is that union of two convex cones may not be convex cone. But I can't understand why? Any thoughts? Thanks

Can you find a counterexample??

Take two cones in real life, is their union a cone?
 
  • #8
OK. Thanks. But if one cone is subset of other then answer would be yes? I am right?
 
  • #9
trenekas said:
OK. Thanks. But if one cone is subset of other when answer would be yes? I am right?

Indeed.
 

1. What is a convex cone?

A convex cone is a set that contains all the points on a line segment connecting any two points within the set. This means that if you pick any two points within the set, all the points on the line connecting them will also be included in the set.

2. What does it mean to prove that the intersection of convex cones is convex?

Proving that the intersection of convex cones is convex means showing that if you have two or more convex cones, the set of points that are contained in all of these cones is also a convex cone. In other words, the intersection of convex cones is also a convex cone.

3. Why is it important to prove that the intersection of convex cones is convex?

Proving that the intersection of convex cones is convex is important because it helps us understand the behavior of convex cones. It also allows us to use convex cones in various mathematical and scientific applications with the assurance that their intersection will also be a convex cone.

4. How do you prove that the intersection of convex cones is convex?

To prove that the intersection of convex cones is convex, you can use the definition of a convex cone and show that the set of points contained in all of the given convex cones also satisfies this definition. This can be done by using mathematical proofs and properties of convex cones.

5. Are there any real-world applications of proving the intersection of convex cones is convex?

Yes, there are many real-world applications of proving the intersection of convex cones is convex. For example, in optimization problems, convex cones are often used to model constraints. By proving the intersection of these cones is convex, we can ensure that the solution space of the problem is also a convex set, making it easier to find an optimal solution. Convex cones are also used in economics, engineering, and statistics, among other fields.

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