Coefficients of Product of Two Polynomials

Theorem
Let $R$ be a commutative ring with unity.

Let $f, g \in R[x]$ be polynomials over $R$.

For a natural number $n \geq 0$, let:
 * $a_n$ be the coefficient of the monomial $x^n$ in $f$.
 * $b_n$ be the coefficient of the monomial $x^n$ in $g$.

As an indexed summation
The coefficient $c_n$ of $x^n$ in $fg$ is the sum:
 * $c_n = \displaystyle \sum_{k \mathop = 0}^n a_k b_{n-k}$

As an indexed summation bounded by degrees
Let $\deg f$ and $\deg g$ be their degrees.

The coefficient $c_n$ of $x^n$ in $fg$ is the sum:
 * $c_n = \displaystyle \sum_{k \mathop = n - \deg g}^{\deg f} a_k b_{n-k}$

Also see

 * Degree of Product of Polynomials