Set of Polynomials over Integral Domain is Subring
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Theorem
Let $\struct {R, +, \circ}$ be a commutative ring.
Let $\struct {D, +, \circ}$ be an integral subdomain of $R$.
Then $\forall x \in R$, the set $D \sqbrk x$ of polynomials in $x$ over $D$ is a subring of $R$.
Proof
By application of the Subring Test:
As $D$ is an integral domain, it has a unity $1_D$ and so $x = 1_D x$.
Hence $x \in D \sqbrk x$ and so $D \sqbrk x \ne \O$.
Let $p, q \in D \sqbrk x$.
Then let:
- $\ds p = \sum_{k \mathop = 0}^m a_k \circ x^k, q = \sum_{k \mathop = 0}^n b_k \circ x^k$
Thus:
- $\ds -q = -\sum_{k \mathop = 0}^n b_k \circ x^k = \sum_{k \mathop = 0}^n \paren {-b_k} \circ x^k$
and so:
- $q \in D \sqbrk x$
Thus as Polynomials Closed under Addition, it follows that:
- $p + \paren {-q} \in D \sqbrk x$
Finally, from Polynomials Closed under Ring Product, we have that $p \circ q \in D \sqbrk x$.
All the criteria of the Subring Test are satisfied.
Hence the result.
$\blacksquare$
Sources
- 1978: Thomas A. Whitelaw: An Introduction to Abstract Algebra ... (previous) ... (next): $\S 64.1$ Polynomial rings over an integral domain