# Chapter 1: Polynomial Rings

This chapter was presented by Tim Seynnaeve and Zachary Adams. All their codes are in Macaulay2.

Example 1.18: elimination of variables (see also chapter 4)

```
restart
R=QQ[x,y,z,MonomialOrder=>Lex]
I=ideal(y-x^3+4*x,z-x^3-x+1)
gens gb I
J=ideal(x^3-7,y^4-5,z-x-y)
flatten entries gens gb J
```

Example 1.18: finitely many points

```
restart
R=QQ[x,y,z]
I=ideal(x*y-z,x*z-y,y*z-x)
I==radical(I)
G=gens gb I
dim I
degree I
--This suggests that V(I) consists of 5 points
--We can find the 5 points using primary decomposition
primaryDecomposition I
```

Example 1.19 with degree reverse lexicographic order

```
restart
R=QQ[x_1..x_4]
--If no monomial order is specified, Macaulay2 chooses DegRevLex by default
f=random(4,R)
g=random(4,R)
I=ideal(f,g)
G=gens gb I
degrees G
--Just out of curiosity, we also compute dimension and degree
dim I
degree I
--Geometric interpretation: Our 2 quartics cut out a 2-dimensional surface of degree 16(=4x4) in C^4
```

Example 1.19 with degree lexicographic order

```
restart
R=ZZ/32003[x_1..x_4,MonomialOrder=>GLex]
--Over QQ the computation takes a long time, so instead we took a finite field
f=random(4,R)
g=random(4,R)
I=ideal(f,g)
G=gens gb I;
degrees G
```