Question

Show that if \(R\) is non-trivial, then the \(R\) -module \(R[X]\) is not finitely generated.

Short answer

Question: Prove that if \(R\) is a non-trivial ring, the \(R\)-module \(R[X]\) is not finitely generated. Answer: This can be shown by creating a polynomial, such as \(g(X) = 1 + X + X^2 + \cdots + X^n\), that cannot be a linear combination of any assumed finite set of generators. The contradiction proves that \(R[X]\) is not finitely generated if \(R\) is non-trivial.

Step-by-step solution

Definitions and Assumptions

Recall the definitions of a non-trivial ring and an \(R\)-module. A non-trivial ring \(R\) is a ring with at least two distinct elements, and an \(R\)-module is an abelian group \(M\) with a scalar multiplication operation \(R \times M \to M\) that satisfies certain properties (associativity, distributivity, etc.). In this case, \(M = R[X]\), which is the set of all polynomials with coefficients in \(R\). Let's assume that \(R[X]\) is finitely generated.

Identify the Finite Set of Generators

Let \(S = \{f_1(X), f_2(X), ... , f_n(X) \}\) be the finite set of generators for the \(R\)-module \(R[X]\).

Create a Polynomial Not in the Span of the Set of Generators

Let's construct a polynomial \(g(X) = 1 + X + X^2 + \cdots + X^{n}\), where all coefficients are the identity element of the ring \(R\). In other words, the polynomial \(g(X)\) has degree \(n\) and its coefficients are all \(1_R\).

Show that \(g(X)\) Cannot be a Linear Combination of the Generators

Let's try to express \(g(X)\) as a linear combination of the generators in \(S\). For \(g(X)\) to be a linear combination, there must exist \(r_1, r_2, \dots, r_n \in R\) such that: \(g(X) = r_1 f_1(X) + r_2 f_2(X) + \cdots + r_n f_n(X)\). However, notice that all the generators \(f_i(X)\) have strictly smaller degree than that of \(g(X)\). Therefore, any linear combination of the generators cannot have a degree equal to or greater than \(n\). This means that \(g(X)\) cannot be a linear combination of the generators in \(S\).

Arrive at a Contradiction and Conclude

We now have a polynomial \(g(X)\) that is not a linear combination of the generators in \(S\). This contradicts our initial assumption that \(R[X]\) is finitely generated since there exists a polynomial in \(R[X]\) that is not expressible as a linear combination of the generators. The assumption of \(R[X]\) being finitely generated must be false. Therefore, the \(R\)-module \(R[X]\) is not finitely generated if \(R\) is non-trivial.

Key concepts

R-module

An \(R\)-module can be imagined as a mathematical structure that generalizes the idea of vector spaces, but instead of using a field of scalars, it employs a ring. Think of a ring \(R\) as a set equipped with two operations: addition and multiplication, which have similar properties to the familiar numbers. In essence, an \(R\)-module \(M\) becomes a collection that allows you to multiply its elements by elements of \(R\) and retain certain properties like associativity and distributivity.

• Associativity: \((r \cdot s) \cdot m = r \cdot (s \cdot m)\)
• Distributivity: \(r \cdot (m_1 + m_2) = r \cdot m_1 + r \cdot m_2\)
• Distributivity in rings: \((r_1 + r_2) \cdot m = r_1 \cdot m + r_2 \cdot m\)

For instance, if \(R\) is a non-trivial ring (meaning it has at least two different elements), the \(R\)-module \(R[X]\) would consist of all polynomials with coefficients in \(R\). This collection is a mix between elements from the ring \(R\) and polynomials that stretch over potentially infinite horizons.

Polynomial

Polynomials are fundamental mathematical expressions involving a sum of powers in one or more variables multiplied by coefficients. Frequently, these coefficients come from a designated ring, such as the integers or real numbers.
To make this concrete, consider a polynomial like \(f(X) = a_n X^n + a_{n-1} X^{n-1} + \cdots + a_1 X + a_0\) where \(a_i\) are elements of the ring \(R\), and \(X\) represents the variable. Each \(a_i X^i\) is a monomial, and combining them gives you the polynomial.

• Degree: The highest power of \(X\) in a polynomial (like \(n\) in our example) is known as the degree of the polynomial.
• Coefficients: These are values from \(R\), which multiply the powers of \(X\).
• Variable: Usually indicated by \(X\), it's the part of the polynomial that can vary.

Under the hood, when we're looking at an \(R\)-module \(R[X]\), it is composed entirely of polynomials where \(X\) is considered the central variable and \(R\) provides the coefficients.

Finitely Generated Module

A finitely generated module is a concept within abstract algebra where a module is produced by a finite set of generators. These generators are elements that, through the operations defined in the module, can produce every element of the module.
To visualize: if we say that an \(R\)-module is finitely generated, we mean there exist a finite set of elements \(\{f_1, f_2, \ldots, f_n\}\) so that every element in the module can be expressed as an \(R\)-linear combination of these generators:
\[ m = r_1 f_1 + r_2 f_2 + \cdots + r_n f_n \] where \(r_i\) are elements from \(R\). But, as shown in our problem, if \(R\) is a non-trivial ring, \(R[X]\) (the polynomial ring) cannot be finitely generated. This arises because for any supposed finite collection of polynomials, you can construct a new polynomial (like \(g(X) = 1 + X + X^2 + \cdots + X^n\)) which falls outside the scope of their linear combinations, pointing out the richness and infinite nature of polynomial expressions.