# Homework Help: Prove intersection of convex cones is convex.

1. Oct 8, 2011

### jvt05

1. Let A and B be convex cones in a real vector space V. Show that A$\bigcap$B and A + B are also convex cones.

2. Oct 8, 2011

### Dick

First state the definition of a convex cone, and then try to prove it. People will help if you show some effort first.

3. Oct 8, 2011

### jvt05

well set C is a convex cone if for any x,y $\in$ C and any scalars a≥0, b≥0, ax + by $\in$ C

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

4. Oct 8, 2011

### jvt05

other properties of convex cones:

1. for any positive scalar α and any x $\in$ 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. Oct 8, 2011

### Dick

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. Apr 15, 2013

### trenekas

Hello. I dont 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 whats about union. The answer is that union of two convex cones may not be convex cone. But I cant understand why? Any thoughts? Thanks

7. Apr 15, 2013

### micromass

Can you find a counterexample??

Take two cones in real life, is their union a cone?

8. Apr 15, 2013

### trenekas

OK. Thanks. But if one cone is subset of other then answer would be yes? Im right?

9. Apr 15, 2013

Indeed.