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Topic: How many concave quadrilaterals are in a 3 by n array?
Replies: 9   Last Post: Sep 27, 2006 10:37 AM

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Ulrich Kortenkamp

Posts: 18
Registered: 12/11/04
Re: How many concave quadrilaterals are in a 3 by n array?
Posted: Sep 11, 2006 4:39 AM
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On 11.09.2006, at 08:55, Oliver Stemforn wrote:

> Ulli,
>
> it is NOT an ambiguous question.


Actually it was to me, otherwise I wouldn't ask.

> 1) I stated that it is a 3 by n array, hence 3n dots.
>


Well, for those who work with homogeneous coordinates, 3 coordinates
for a single point is not really surprising. And as those people,
including me, usually write down n points as an 3 by n matrix, it's
better to ask, isn't it?

> 2) I stated that they are "in one plane" so there is no 3-space to
> consider.


Again, there is no problem with having a plane in 3-space. And if you
think projectively, 3-space isn't that far from the projective plane.

Now, let me ask another question, because I still don't understand
your question: What are the entries of the 3 by n matrix? What
exactly do you mean by "dot"?

Ulli




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