The Pippenger product is an unexpected Wallis-like formula for e given by
 |
(1) |
(Sloane's A084148 and A084149; Pippenger 1980). Here, the nth term for 
is given by
 |
 |
 |
(2) |
|
 |
(3) |
where
is a double factorial and 
is the gamma function.
Pippenger, N. "An Infinite Product for e." Amer. Math. Monthly 87, 391, 1980.
Sloane, N. J. A. Sequences A084148 and A084149 in "The On-Line Encyclopedia of Integer Sequences." http://www.research.att.com/~njas/sequences/.