Dilogarithm

The dilogarithm is a special case of the polylogarithm for . Note that the notation is unfortunately similar to that for the logarithmic integral . There are also two different commonly encountered normalizations for the function, both denoted , and one of which is known as the Rogers L-function.

The dilogarithm is implemented in the Wolfram Language as PolyLog[2, z].

The dilogarithm can be defined by the sum

(1)

or the integral

(2)

Plots of in the complex plane are illustrated above.

The major functional equations for the dilogarithm are given by

			(3)
			(4)
			(5)
			(6)

A complete list of with real arguments that can be evaluated in closed form is given by

			(7)
			(8)
			(9)
			(10)
			(11)
			(12)
			(13)
			(14)
			(15)
			(16)
			(17)
			(18)

where is the golden ratio (Lewin 1981, Bailey et al. 1997; Borwein et al. 2001).

Two-term identities involving irrational numbers include

(19)

(Lima 2012, Campbell 2021) and

			(20)
			(21)
			(22)
			(23)

where is the golden ratio, is Catalan's constant, and is the trigamma function (Campbell 2021).

W. Gosper (Sep. 19, 2021) gave the following identity for a dilogarithm with complex argument

(24)

where denotes the imaginary part of and is the trigamma function.

There are several remarkable identities involving the dilogarithm function. Ramanujan gave the identities

			(25)
			(26)
			(27)
			(28)
			(29)

(Berndt 1994, Gordon and McIntosh 1997) in addition to the identity for , and Bailey et al. (1997) showed that

(30)

Lewin (1991) gives 67 dilogarithm identities (known as "ladders"), and Bailey and Broadhurst (1999, 2001) found the amazing additional dilogarithm identity

(31)

where is the largest positive root of the polynomial in Lehmer's Mahler measure problem and is the Riemann zeta function.