Exercises: Ideals , Extensions & Vector Spaces
Exercise
Let us start with an exercise that bridges the concepts of ideals , simple extension fields,
and introduces vector spaces.
Exercise:Exploring a field E as a Vector Space.
Let F = `QQ` be the field of rational numbers. Consider the polynomial
`p(x) = x^2 - 2` `in QQ[x]`.
1. Show that the ideal `I = langle p(x) rangle` generated by p(x) in `QQ[x]` is a maximal ideal.
2. Deduce that the quotient ring `K = QQ[x]"/" I` is a field. This is a simple extension field of `QQ`.
3. Demonstrate that K is a vector space over `QQ`.
4. Find a basis for K as a vector space over `QQ` and determine its dimension.
Detailed Solution (including explanation of Vector Space theory used):
Before we dive into the solution , let us briefly revisit the definition of a Vector Space.
Introduction to Vector Spaces:
A vector space (or linear space) V over a field F is a set V equipped with
two operations:
1. Vector Addition: For any u , v `bb in V` , there is a unique element `bb {u + v in V}`.
2. Scalar Multiplication: For any c `in F` and u `bb {in V}` , there is a unique element
`c bb {u in V}`.
Continuing:
Solution:
Let us adress each part of the exercise:
1. Show that the ideal `I = langle p(x) rangle` generated by
`p(x) = x^2 - 2 in QQ[x]` is a maximal ideal.
In a polynomial ring F[x] over a field F , an ideal `langle f(x) rangle` is maximal
if and only if f(x) is an irreducible polynomial over F. So we need to show that
`p(x) = x^2 - 2` is irreducible over `QQ`.
Irreducibility Test: For a polynomial of degree 2 or 3 , it is irreducible
if and only if
it has a root in `QQ`. Let us check if `x^2 - 2` has any rational roots.
If r is a rational root , then `r^2 = 2`. Suppose `r = a/b` where a , b `in ZZ` ,
`b ne 0` and gcd(a , b) = 1. Then `(a/b)^2 = 2 rArr a^2/b^2 = 2 rArr a^2 = 2b^2`.
This implies that `a^2` is an even number , which means a is an even number. Let a = 2k
for some integer k. Substituting a = 2k into the equation: `(2k)^2 = 2b^2 rArr
4k^2 = 2b^2 rArr 2k^2 = b^2`. This implies that `b^2` is an even number , which means
b must be an even number.However , if both a and b are even , then gcd(a , b)`le` 2 ,
which contradicts our assumption that gcd(a , b) = 1. Therefore , `x^2 - 2` has no rational
roots. Since `x^2 - 2` is a polynomial of degree 2 and has no rational roots , it is
irredicible over `QQ`. Thus , the ideal `bb {I = langle x^2 - 2rangle}` is a
maximal ideal in `QQ[x]`. QED
Question
Which of the following statement is correct regarding the polynomial `p(x) = x^2 - 2`:
A) The polynomial `p(x) = x^2 - 2` is irreducible over `QQ` because it has no rational roots.
B) The polynomial `p(x)= x^2 - 2` is reducible over `QQ` because it has roots , namely
`sqrt 2` and `- sqrt 2`
?