# Definition:Wallis's Number

## Definition

Wallis's number is the real root of the cubic:

$x^3 - 2 x - 5 = 0$

Its approximate value is:

$2 \cdotp 09455 \, 1 \ldots$

## Also known as

The name of this number is also presented as Wallis' number.

## Source of Name

This entry was named for John Wallis.

## Historical Note

The cubic $x^3 - 2 x - 5 = 0$ was used by John Wallis as a example to demonstrate how Newton's Method could be used to solve certain equations numerically.

The real root of this equation has since become known as Wallis's number.

It has subsequently been used as a test for a number of different methods of approximation.

It is known to over $4000$ digits.