Basic thing of Conic in Projective Geometry

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Discussion Overview

The discussion revolves around the concept of conics in projective geometry, contrasting it with their representation in Euclidean geometry. Participants explore the properties of conics, including their definitions, the nature of homogeneous equations, and the distinction between degenerate and non-degenerate conics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant introduces the definition of a conic as a subset of P2 defined by a homogeneous quadratic equation and questions the nature of homogeneity.
  • Another participant explains that an equation is homogeneous if all terms have the same degree, providing examples of terms with degree 2.
  • There is a query about why certain terms (dXY, eXZ, fYZ) can be eliminated in suitable coordinates, with a suggestion that coordinate changes can facilitate this reduction.
  • Participants discuss the difference between degenerate and non-degenerate conics, with examples illustrating that degenerate conics consist of lines and points, while non-degenerate conics are smooth curves.
  • One participant emphasizes the importance of visualizing conics geometrically rather than solely relying on equations, sharing a drawing to aid understanding.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and approaches to the topic, but there is no explicit consensus on the nuances of the definitions or the implications of the properties discussed.

Contextual Notes

The discussion includes assumptions about the nature of conics and their representations, but these assumptions are not universally agreed upon. The exploration of coordinate transformations and their effects on the conic equations remains unresolved.

Who May Find This Useful

This discussion may be useful for students and enthusiasts of projective geometry, particularly those interested in the properties and visual representations of conics.

wawar05
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I am new about conic in projective geometry since it seems to be really different in euclidean plane.

A conic is a subset of P2 given by a homogenous quadratic equation:

aX^2 + bY^2 + cZ^2 + dXY + eXZ + fYZ = 0

why is it homogeneous?

meanwhile, it suitable coordinates we have aX^2 + bY^2 + cZ^2 = 0, with a, b, c element {0, 1, -1}.

why the part of dXY + eXZ + fYZ can be erased?

what is the difference of degenerate conic and non-degenerate conic?
 
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wawar05 said:
I am new about conic in projective geometry since it seems to be really different in euclidean plane.

A conic is a subset of P2 given by a homogenous quadratic equation:

aX^2 + bY^2 + cZ^2 + dXY + eXZ + fYZ = 0

why is it homogeneous?

An equation is homogeneous if all the terms have the same degree. A term of the form X2 or XY all have degree 2.

meanwhile, it suitable coordinates we have aX^2 + bY^2 + cZ^2 = 0, with a, b, c element {0, 1, -1}.

why the part of dXY + eXZ + fYZ can be erased?

The point is that you can change coordinates in such a way such that the d, e and f can be dropped. See http://home.scarlet.be/~ping1339/reduc.htm for a reduction of a conic section to it's reduced form.

what is the difference of degenerate conic and non-degenerate conic?

A degenerate conic consists of lines and points, whilme a non-degenerate conic is a nice curve.

For example, the conic

aX^2+bY^2=0

has only (0,0) as a solution, thus the conic is just a point. The conic

X^2+2XY+Y^2=0

is the same as

(X+Y)(X+Y)=0

Thus the conic is two times the line X=-Y. Such a conics are degenerate because they are not the nice smooth curves we expect.
 
I always find it helpful to go back to the basics when thinking about the conics. the geometry of it all is much simpler than equations and is SO easy to visualize. I've done up a drawing for you:
 

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^^, thank you for the helps... I am now having good understanding of conic related to projective plane...
 

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