# Definition:Convergent Product/Normed Algebra

## Definition

Let $\mathbb K$ be a division ring with norm $\norm{\,\cdot\,}_{\mathbb K}$.

Let $\struct{ A, \norm{\,\cdot\,} }$ be an associative normed unital algebra over $\mathbb K$.

Let $\sequence{a_n}$ be a sequence in $A$.

### Definition 1

The infinite product $\displaystyle \prod_{n \mathop = 1}^\infty a_n$ is **convergent** if and only if there exists $n_0\in\N$ such that:

- $a_n$ is invertible for $n \geq n_0$
- the sequence of partial products of $\displaystyle \prod_{n \mathop = n_0}^\infty a_n$ converges to some invertible $b\in A^\times$.

### Definition 2: for complete algebras

Let $\struct{ A, \norm{\,\cdot\,} }$ be complete.

The infinite product $\displaystyle \prod_{n \mathop = 1}^\infty a_n$ is **convergent** if and only if there exists $n_0\in\N$ such that:

- the sequence of partial products of $\displaystyle \prod_{n \mathop = n_0}^\infty a_n$ converges to some invertible $a\in A^\times$.