For a finite sequence a1, a2,…, an, the product a1 a2…an may be denoted, using the capital Greek letter pi, by
where r is a dummy variable. For example,
Similarly, from an infinite sequence a1, a2, a3,…, an infinite product a1a2a3…can be formed, and it is denoted by
This notation is also used for the limit of the infinite product, if it exists. For example
and also as in Wallis’ product.