#0, Projective Geometry
Posted by gd88 on Mar-29-08 at 11:28 AM
The Pojective Geometry is a "Non Euclidean Geometry"? When I say "Non Euclidean Geometry" I think Hiperbolic Geometry or Elliptic Geometry: Projective Geometry belongs to this set of geometry or not?Thanks
#1, RE: Projective Geometry
Posted by alexb on Mar-29-08 at 11:38 AM
In response to message #0
Absolutely! Projective geometry in which any two lines intersect is non-Euclidean.Hyperbolic and Elliptic geometries came from different interpretations of the Fifth Postulate and this is why the are commonly labeled non-Euclidean. Projective geometry was an independent development but is as much non-Euclidean as the other two. In addition, any non-discrete geometry is also non-Euclidean. So geometries may be non-Euclidean but for different reasons. Quite often the Taxicab geometry is presented as one of the variety. I believe it's a misnomer. In fact, I do not think it's a geometry at all. It's a metric space, yes, but not a geometry. It violates the most common requirement of having a single line through any two points.
#2, RE: Projective Geometry
Posted by gd88 on Mar-29-08 at 12:20 PM
In response to message #1
Ok. Projective Geometry, Hyperbolic Geometry and Elliptic Gemetry are non-Euclidean Geometry.And Affine Geometry? Is Affine Geometry a non-Euclidean Geometry? P.S. I'm sorry for the quality of my questions, but I can't speak english very well.
#3, RE: Projective Geometry
Posted by alexb on Mar-29-08 at 12:33 PM
In response to message #2
Affine geometry is also non-Euclidean. E.g., in affine geometry the is no notion of angle measure. There are no right angles.Euclidean geometry is based on five postulates and some common notions. There are a few axioms Euclid assumed implicitly without mentioning, e.g. that a straight line cuts the plane into two parts such that is a point from one is joint to a point on the other, the segment will cross the line. Or that two circles under certain conditions intersect in two points. You should look whether a geometry violates any of the five postulates, although, as I said, I would not call "a geometry" a theory that allows more than one line through two points.
#4, RE: Projective Geometry
Posted by gd88 on Mar-29-08 at 12:50 PM
In response to message #3
Ok. Thanks very much.
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