# Polynomial systems

Chapter 9 of the open-source book *Calcul mathématique avec Sage* (in French)
is about polynomial systems. In particular, check section 9.2. The book is
available for free download from:
http://sagebook.gforge.inria.fr/ (click
"Telecharger le PDF").

The answer below closely follows that reference, with minor adaptations in
order to address the ask-sage question by MvG.

Credit goes to Marc
Mezzaroba who authored that chapter, and more generally to the team who
authored the book and kindly provides it under a Creative Commons
license
allowing all to copy and redistribute the material in any medium or format,
and to remix, transform, and build upon the material, for any purpose.

## The system

In section 9.2.1, the following polynomial system is considered:

$$
\left \{ \quad
\begin{array}{@{}ccc@{}} x^2 \; y \; z & = & 18 \\
x \; y^3 \; z & = & 24\\
x \; y \; z^4 & = & 6 \\
\end{array}\right.
$$

## Numerical solve vs algebraic approach

While section 2.2 of the book explained how to solve numerically with `solve`

,

```
sage: x, y, z = var('x, y, z')
sage: solve([x^2 * y * z == 18, x * y^3 * z == 24,\
....: x * y * z^4 == 3], x, y, z)
[[x == (-2.76736473308 - 1.71347969911*I), y == (-0.570103503963 +
2.00370597877*I), z == (-0.801684337646 - 0.14986077496*I)], ...]
```

section 9.2.1 explains how to solve algebraically.

## Ideal in a polynomial ring

First translate the
problem in more algebraic terms: we are looking for the common zeros
of three polynomials, so we consider the polynomial ring over `QQ`

in
three variables, and in this ring we consider the ideal generated by
the three polynomials whose common zeros we are looking for.

```
sage: R.<x,y,z> = QQ[]
sage: J = R.ideal(x^2 * y * z - 18,
....: x * y^3 * z - 24,
....: x * y * z^4 - 6)
```

We check that the dimension of this ideal is zero, which means the system
has finitely many solutions.

```
sage: J.dimension()
0
```

## Solution, algebraic variety, choice of base ring

The command `variety`

will compute all the solutions of the system.
However, its default behaviour is to give the solutions in the base ring
of the polynomial ring. Here, this means it gives only the rational
solutions.

```
sage: J.variety()
[{y: 2, z: 1, x: 3}]
```

We want to enumerate the complex solutions, as exact algebraic numbers.
To do that, we use the field of algebraic numbers, `QQbar`

. We find the 17
solutions (which were revealed by the numerical approach with `solve`

).

```
sage: V = J.variety(QQbar)
sage: len(V)
17
```

Here is what the last three solutions look like as complex numbers.

```
sage: V[-3:]
[{z: 0.9324722294043558? - 0.3612416661871530?*I,
y: -1.700434271459229? + 1.052864325754712?*I,
x: 1.337215067329615? - 2.685489874065187?*I},
{z: 0.9324722294043558? + 0.3612416661871530?*I,
y: -1.700434271459229? - 1.052864325754712?*I,
x: 1.337215067329615? + 2.685489874065187?*I},
{z: 1, y: 2, x: 3}]
```

Each solution is given as a dictionary, whose keys are the generators of
`QQbar['x,y,z']`

(and not `QQ['x ...`

(more)