# Basic thing of Conic in Projective Geometry

1. Jun 29, 2011

### wawar05

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?

2. Jun 29, 2011

### micromass

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

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.

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.

3. Jun 29, 2011

### phinds

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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4. Jun 30, 2011

### wawar05

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