# Projective Plane .. Cox et al - Section 8.1, Exs 5(a) & 5(b)

1. Jul 19, 2016

### Math Amateur

1. The problem statement, all variables and given/known data

I am reading the undergraduate introduction to algebraic geometry entitled "Ideals, Varieties and Algorithms: An introduction to Computational Algebraic Geometry and Commutative Algebra (Third Edition) by David Cox, John Little and Donal O'Shea ... ...

I am currently focused on Chapter 8, Section 1: The Projective Plane ... ... and need help getting started with Exercises 5(a) and 5(b) ... ...

Exercise 5 in Section 8.1 reads as follows:

2. Relevant equations

The definitions 1, 2 and 3 in Cox et al Section 8.1 may be relevant ... see the text provided below ...
...

3. The attempt at a solution

I am very unsure how to start on this exercise ... but i suspect that the following transformation or map as given in Cox et al Section 8.1 directly after Definition 3 is involved:

$\mathbb{R}^2 \longrightarrow \mathbb{P}^2 ( \mathbb{R} )$

where $(x, y) \in \mathbb{R}^2$ is sent to the point $p \in \mathbb{P}^2 ( \mathbb{R} )$ whose homogeneous coordinates are $(x, y, 1)$

Hope someone can help ...

Peter

======================================================================

To give readers of the above post some idea of the context of the exercise and also the notation I am providing some relevant text from Cox et al ... ... as follows:

#### Attached Files:

File size:
42.6 KB
Views:
92
File size:
151.3 KB
Views:
94
File size:
58.4 KB
Views:
99
File size:
108.6 KB
Views:
100
File size:
94.2 KB
Views:
100
File size:
101 KB
Views:
97
• ###### Cox et al ...- 6 - The Projective Plane - page 6 ... ....png
File size:
98.7 KB
Views:
99
Last edited: Jul 19, 2016
2. Jul 19, 2016

### andrewkirk

I can have a go at (a).
We want the equation to be compatible with the equation $y=x^2$ and we also want it to give a well-defined curve, which means it must be homogeneous in x,y and z.

A simple equation that satisfies both those is $yz=x^2$. Then for $z=1$ this gives the original equation. Any point in $\mathbb R^2$ with nonzero $z$ is the same as a point with $z=1$. The only other points are those with $z=0$, which are at infinity. For such points we will also have, courtesy of the equation, $x=0$. So the set of points on the curve at infinity are those on the y axis in $\mathbb R^2$. This comprises two equivalence classes: [(0,0,0)] and [(0,1,0)]. So there are two points at infinity, which sounds like what we would want for a parabola (which answers part (b)).

So far so good. I haven't thought about c or d yet.

3. Jul 19, 2016

### Math Amateur

Thanks for the help Andrew ... but ... your solution seems to have been achieved with some good insight ...

Is there a method or process for transforming curves in $\mathbb{R}^2$ to curves in $\mathbb{P}^2 ( \mathbb{R} )$?

Thanks again ...

Peter

P.S. if you have any ideas about parts (c) and (d) please let me know ...

4. Jul 19, 2016

### andrewkirk

I don't know of any general method but, for curves whose equations are polynomial in x and y, the method is easy enough. You just write out the equation and find the degree of each term, which is the exponent for x plus the exponent for y. Let M be the highest such degree in the equation. Then, to make the equation homogeneous, we multiply each term by $z^{M-d}$ where $d$ is the degree of the term. We then have an equation that is homogeneous - hence making a well-defined curve in the projective plane - and which also involves $z$.

There are a couple of examples of this technique in the author's discussion following Proposition 4.

(d) is easy. Just set $x$ equal to any nonzero constant, such as 1. The equation then becomes $yz=1$, which is the classic hyperbola equation.

To do (c) we'd need to understand what the author means by 'tangent to' in the context of a projective plane. Do you know what he means by that?

5. Jul 22, 2016

### Math Amateur

Thanks Andrew ...

By the way, your approach seems to be a general method to extend algebraic curves in the Euclidean plane to the Projective plane ... the book: "Conics and Cubics: A Concrete Introduction to Algebraic Curves"by Robert Bix outlines the method as follows:

File size:
98.2 KB
Views:
62
File size:
90.5 KB
Views:
63
File size:
100.5 KB
Views:
66