Polynomial Addition is Associative

Theorem
Addition of polynomials is an associative operation.

Proof
Let $\struct {R, +, \circ}$ be a commutative ring with unity.

Let $\set {X_j: j \in J}$ be a set of indeterminates.

Let $Z$ be the set of all multiindices indexed by $\set {X_j: j \in J}$.

Let:


 * $\ds f = \sum_{k \mathop \in Z} a_k \mathbf X^k$


 * $\ds g = \sum_{k \mathop \in Z} b_k \mathbf X^k$


 * $\ds h = \sum_{k \mathop \in Z} c_k \mathbf X^k$

be arbitrary polynomials in the indeterminates $\set {X_j: j \in J}$ over $R$.

Then: