Wallis Formula

a formula that expresses the number 4/π in the form of an infinite product:

The Wallis formula is not well suited to the direct calculation of π but it is useful in various theoretical discussions—for example, in the derivation of Stirling’s formula. The Wallis formula was historically significant as one of the first examples of infinite products. J. Wallis arrived at it in 1665 while calculating the area of a circle.