While (1) and (2) seem unrelated, the above expression (3) shows that they are both
special cases of a more general “double product”. The first product in (3) consists of the
first p factors of Vieta’s original infinite product (1). The second product in (3) is a
Wallis-like product. We say this because the case where p = 0 gives us the original
Wallis’s product (2), and for other values of p it is the original Wallis’s product with
factors deleted. Notice also that the Wallis-like product in (3) provides us with the error factor needed to make the Vieta product (1) exact when only a finite number of factors are used .