Factors in Convergent Product Converge to One

Theorem
Let $\struct {\mathbb K, \norm{\,\cdot\,}}$ be a valued field.

Let the infinite product $\displaystyle \prod_{n \mathop = 1}^\infty a_n$ be convergent.

Then $a_n\to0$.

Proof
By definition of convergent product, there exists $n_0\in\N$ such that:
 * $a_n\neq0$ for $n\geq n_0$
 * the sequence of partial products of $\displaystyle \prod_{n \mathop = n_0}^\infty a_n$ has a nonzero limit.

Let $p_n$ denote the $n$th partial product of $\displaystyle \prod_{n \mathop = n_0}^\infty a_n$.

For $n> n_0$, $a_n = \frac{p_{n}}{p_{n-1}}$.

By the Combination Theorem for Sequences, $a_n\to1$.

Also see

 * Factors in Absolutely Convergent Product Converge to One