Is set $\displaystyle \left\{ {A,B} \right\}$,$\displaystyle A \ne B$ convex?

Since there is no AB segment in set then set is concave.

Am I right?

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- Aug 10th 2006, 06:14 AMOReillyConvex set
Is set $\displaystyle \left\{ {A,B} \right\}$,$\displaystyle A \ne B$ convex?

Since there is no AB segment in set then set is concave.

Am I right? - Aug 10th 2006, 06:43 AMCaptainBlackQuote:

Originally Posted by**OReilly**

Is $\displaystyle \left\{ {A,B} \right\}$ a subset of a vector space?

Does it not have to be to be able to ask the question.

If it is; it does not contain any point on the segment $\displaystyle AB$

if $\displaystyle A \ne B$

RonL - Aug 10th 2006, 09:43 AMOReillyQuote:

Originally Posted by**CaptainBlack**

I don't know, I retyped question from book.

In the book there isn't definition of convex set, so I have read definition from one internet site which says that convex set is "a set of points in which all segments connecting points of the set lie entirely in the set".

I didn't understand the question quite so I don't know. - Aug 10th 2006, 10:06 AMCaptainBlackQuote:

Originally Posted by**OReilly**

points in the subset all the points between are also in the subset.

RonL - Aug 10th 2006, 10:16 AMOReillyQuote:

Originally Posted by**CaptainBlack**

- Aug 10th 2006, 12:53 PMJakeDQuote:

Originally Posted by**CaptainBlack**

Quote:

Originally Posted by**OReilly**

Quote:

Originally Posted by**OReilly**

- Aug 10th 2006, 01:04 PMCaptainBlackQuote:

Originally Posted by**JakeD**

us wondering about what it might mean by a segment for an arbitrary

space, and would the definition of convexity work if there were no segment.

Or maybe I am over analysing the question.

Anyway, if they are elements of a vector space there is no problem, the set is

not convex.

RonL - Aug 10th 2006, 03:40 PMOReillyQuote:

Originally Posted by**JakeD**

- Aug 10th 2006, 09:31 PMCaptainBlackQuote:

Originally Posted by**OReilly**

otherwise it is used in that way.

RonL - Aug 10th 2006, 09:36 PMJakeDQuote:

Originally Posted by**OReilly**

- Aug 10th 2006, 10:22 PMCaptainBlackQuote:

Originally Posted by**JakeD**

wrong. The usual usage is that a set which is not convex is concave (a

usage that I don't like but there it is) He seems to think concavity and

convexity are the same thing which is confusing (and if they were defined

that way would be a waste of a good word), and then uses the non-standard

one as his default.

RonL - Aug 11th 2006, 08:37 AMRebesques
...The title of that page goes "

**Concavity (Convexity)**"!

Strange... What about defining "Smoker (Passive Smoker)"??

Is this the case where opposites mix, or just a slip of the fingers on the keyboard? :p

ps ...Well, if concavity and convexity were exactly the same thing, there would be no need to have a definition in the first place. - Aug 11th 2006, 11:45 AMJakeD
How and whether you define concavity of a set is important in economics.

The problem with defining concavity of sets as non-convexity is that this can lead to confusion when defining functions as concave or quasi-concave, which are very important in economics. Functions which are concave or quasi-concave have nice maximums.

A function is concave if for all $\displaystyle t \in (0,1),$ $\displaystyle f(tx + (1-t)y) \ge tf(x)+(1-t)f(y).$ Geometrically, that means the set of points below the graph of $\displaystyle f$ is*convex.*So concavity (non-convexity) of a set has no relation to concavity of a function.

Similarly a function is quasi-concave if the upper level sets $\displaystyle \{x | f(x) \ge y\}$ are convex. Again concavity (non-convexity) of a set has no relation to quasi-concavity of a function.

When teaching these definitions, the important concepts are convexity of set and concavity and quasi-concavity of functions. There is no need to bring in concavity of a set. It just leads to confusion. If you want to say a set is not convex, just say that or use the word non-convex.

What the page I referred to above did was to define concave set in a way that was useful for defining concave functions. At the time it was written (1983), this may have not been incorrect because I don't think the definition of concavity of a set was well-established then. Definitions change over time in mathematics.

Here is a quote from another page google turns up:

I used words like "convex set" and "concave set" in class. It has been pointed out that while "concave set" is a mathematical concept, it is not nearly as commonly used as "convex set", which is the common vocab including in economic applications.

There's good reason for this: In a convex set, if you join 2 points by a line segment, the line segment lies entirely within the set. In a nonconvex (concave) set, if you join 2 points, the line segment may or may not lie within the set, depending on the points you chose. This makes convexity easier to deal with.

He then goes on to discuss quasi-concave functions.