# What is Wallis formula

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## Wallis Formula

The Wallis formula follows from the infinite product representation of the sine

 (1)

Taking gives

 (2)

so

(OEIS A052928 and A063196).

An accelerated product is given by

where

 (7)

(Guillera and Sondow 2005, Sondow 2005). This is analogous to the products

 (8)

and

 (9)

(Sondow 2005).

A derivation of equation (◇) due to Y. L. Yung (pers. comm., 1996; modified by J. Sondow, pers. comm., 2002) defines

where is a polylogarithm and is the Riemann zeta function, which converges for . Taking the derivative of (11) gives

 (13)

which also converges for , and plugging in then gives

Now, taking the derivative of the zeta function expression (◇) gives

 (17)

and again setting yields

where

 (22)

(OEIS A075700) follows from the Hadamard product for the Riemann zeta function. Equating and squaring (◇) and (◇) then gives the Wallis formula.

This derivation of the Wallis formula from using the Hadamard product can also be reversed to derive from the Wallis formula without using the Hadamard product (Sondow 1994).

The Wallis formula can also be expressed as

 (23)

The q-analog of the Wallis formula with is

(OEIS A065446; Finch 2003), where is the q-Pochhammer symbol. This constant is , where is the constant encountered in digital tree searching. The form of the product is exactly the generating function for the partition function P due to Euler, and is related to q-pi.

Portions of this entry contributed by Jonathan Sondow (author's link)

REFERENCES:

Abramowitz, M. and Stegun, I. A. (Eds.). Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables, 9th printing. New York: Dover, p. 258, 1972.

Finch, S. R. "Archimedes' Constant." §1.4 in Mathematical Constants. Cambridge, England: Cambridge University Press, pp. 17-28, 2003.

Guillera, J. and Sondow, J. "Double Integrals and Infinite Products for Some Classical Constants Via Analytic Continuations of Lerch's Transcendent." 16 June 2005. http://arxiv.org/abs/math.NT/0506319.

Jeffreys, H. and Jeffreys, B. S. "Wallis's Formula for ." §15.07 in Methods of Mathematical Physics, 3rd ed. Cambridge, England: Cambridge University Press, p. 468, 1988.

Kenney, J. F. and Keeping, E. S. Mathematics of Statistics, Pt. 2, 2nd ed. Princeton, NJ: Van Nostrand, pp. 63-64, 1951.

Sloane, N. J. A. Sequences A052928, A063196, A065446, and A075700 in "The On-Line Encyclopedia of Integer Sequences."

Sondow, J. "Analytic Continuation of Riemann's Zeta Function and Values at Negative Integers via Euler's Transformation of Series." Proc. Amer. Math. Soc.120, 421-424, 1994.

Sondow, J. "A Faster Product for and a New Integral for ." Amer. Math. Monthly112, 729-734, 2005.

Wallis, J. Arithmetica Infinitorum. Oxford, England, 1656.

Referenced on Wolfram|Alpha: Wallis FormulaCITE THIS AS:

Sondow, Jonathan and Weisstein, Eric W. "Wallis Formula." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/WallisFormula.html

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