1. Introduction
Polylogarithm function
The
polylogarithm function
plays a significant role in many areas of number theory;
its origin, the dilogarithm , dates back to Abel, Euler, Kummer, Landen and Spence etc.
See Kirillov [
1], Lewin [
2], Zagier [
3] for more details.
The main theme of this article is to better
understand the relation between the dilogarithm, trilogarithm functions
and zeta values , (Apéry constant),
in terms of new integral representations.
Main results
First, we wish to briefly explain work of Boo Rim Choe (1987) [
4], Ewell (1990) [
5] and Williams-Yue (1993) [
6, p.1582-1583] which motivated us. Their common idea is that, from Maclaurin series involving , they each derived certain infinite sums related to and with termwise Wallis integral. Figure 1 gives summary of this.
Table 1. Summary of Boo Rim Choe, Ewell and Williams-Yue’s work.
Boo Rim Choe |
|
|
|
Ewell |
|
|
|
Williams-Yue |
|
|
|
In this article, we reformulate their ideas introducing Wallis operator and naturally extend their results.
- We find new integral representations for
, , Legendre functions of order 2, 3 and even for Apéry, Catalan constants (Theorems 2, 5, Corollaries 3, 6).
- We give a lower bound for on the unit interval (Theorem 7).
- Making use of and , we prove new Euler sums (Theorem 8).
Notation
Throughout this paper, denotes a nonnegative integer.
Let
In particular, we understand that .
Moreover, let
Notice the relation as we will see in the sequel.
Remark 1.
- The sequence appears in Wallis integral as
- It also appears in the literature in the disguise of central binomial coefficients as
See Apéry [7], van der Poorten [8], for example.
Unless otherwise specified, are real numbers.
By and , we mean the real inverse sine and cosine functions (), that is,
Fact 1.
(Gradshteyn-Ryzhik [9, p.60, 61])
Further,
denotes the inverse hyperbolic sine function
(some authors write or for this one).
2. Dilogarithm function
2.1. Definition
Definition 1.
For , the dilogarithm function is
In particular,
It is possible to describe its even part by itself since
Its
odd part is called
the Legendre function of order 2:
Here is a fundamental relation of these two parts.
Observation 1.
Definition 2.
Define
as a signed analog of .
This is also called the
inverse tangent integral of order 2 because of the integral representations
2.2. Wallis operator
Let denote the set of power series in
over real coefficients.
Set
Definition 3.
For , define by
Call the Wallis operator.
Remark 2.
[9, p.17] Power series may be integrated and differentiated termwise inside the circle of convergence
without changing the radius of convergence.
In the sequel, we will frequently use this without mentioning explicitly.
It is now helpful to understand coefficientwise.
Lemma 1.
Let .
Then
Proof.
Observe that
is linear in the sense that
and
for .
2.3. Main Theorem 1
Lemma 2.
All of the following are convergent power series for .
Proof.
We already saw (3) and (4) in Introduction.
(5) is .
(6) and (7) follow from
(3), (4)
and (for all )
[9, p.56].
Theorem 2.
For , all of the following hold;
Proof.
Note that these are equivalent to the following statements:
With Lemmas 1 and 2, we can verify (13)-(16) by checking coefficients of those series. For example,
It remains to show (17).
Corollary 3.
Proof.
These are
and
.
3. Trilogarithm function
3.1. Definition
Definition 4. The trilogarithm function for is
Its odd part is the Legendre function of order 3:
In particular, and
.
Observation 4.
Further, a signed analog of is
3.2. Main Theorem 2
Lemma 3.
Proof.
We can derive all of these
by integrating (3)-(7) termwise.
As a consequence, we obtain the
equalities below (cf. (13)-(17)).
In this way, the five functions above come to possess
double integral representations.
For example,
We can indeed simplify such integrals to
single ones by exchanging order of integrals.
Theorem 5.
Proof.
We give a proof altogether.
For , all the equalities hold as .
Suppose . Let
Then
Corollary 6.
Proof.
These are
and
.
4. Applications
4.1. Inequalities
It is easy to see from
the definitions
and
()
that
In fact, we can improve these inequalities a little more.
For upper bounds, it is immediate that
We next prove nontrivial lower bounds for these functions and also .
Theorem 7.
For ,
Before the proof, we need a lemma. It provides another integral representation of which seems interesting itself.
Lemma 4.
For ,
Proof.
If , then both sides are . For ,
Proof of Theorem 7.
If , then all of (43)-(45) hold as .
Suppose .
Since is increasing on , for all .
Then
Next, we prove (44). Note that
for .
Integrate these from to in so that
Quite similarly,
for , it also holds that
The left hand side is
4.2. Euler sums
Definition 5.
A harmonic number is .
More generally, for ,
an - harmonic number is
In particular, .
Any series involving such numbers is called an
Euler sum.
Valean [10, p.292-293] presents truly remarkable Euler sums such as
There are many ideas to prove such formulas;
Borwein and Bradley [
11]
gives thirty two proofs for
by integrals, polylogarithm functions, Fourier series and hypergeometric functions etc.
Here, as an application of our main idea, Wallis operators, we prove two new Euler sums.
Let
Theorem 8.
For the proof, we make use of less-known
Maclaurin series for and
; thus we can interpret this result as a natural subsequence of Boo, Ewell and Williams-Yue’s work.
Lemma 5.
.
Proof.
First, write
, and
let .
It is enough to show that .
Since
the series starts from the term, .
For convenience, set
Then
Now let (),
.
Recall that
In terms of , , this is
Thus,
Differentiate both sides twice in :
Equating coefficients of yields
Since and , we must have
With , we now arrive at
as required.
The proof for (50) proceeds along the same line.
Write
, and
let .
It is enough to show that .
Since
the series starts from the term, .
For convenience, set
Then
Now let (),
.
Recall that
In terms of , , this is
Thus,
Differentiate both sides twice in :
Equating coefficients of yields
Since and , we must have
With , we conclude that
Proof of Theorem 8.
Note that
Clearly, gives the sum for (47).
Therefore,
Similarly, we have
so that
We conclude that
Remark 3.
-
(47) is a variation of De Doelder’s formula
[12, p.1196 (13)]
and (48) gives another proof of
[10, p.286]
because
- After preparation of the manuscript, Christophie Vignat kindly told me that recently Guo-Lim-Qi (2021) [13] described
Maclaurin series of integer powers of arcsin.
In fact, it was the result from J.M. Borwein-Chamberland (2007) [14].
4.3. Integral evaluation
As byproduct of our discussions, we find evaluation of many integrals with known special values of .
Here, we record several examples.
Let be the golden ratio. Observe that
We write for .
Note that
Fact 2.
([2]).
Corollary 9.
Proof.
(34) for with (53)
gives (58).
Finally, (34) for with (55)
gives (60).
5. Concluding remarks
Here, we record several remarks for our future research.
- For , define a generalized Wallis operator
so that we can deal with more general integrals.
Study , particularly for .
- Can we show any inequality for and in a similar way?
- Discuss , and related Euler sums.
- Wolfram alpha [15] says that
It should be possible to describe such integrals as certain infinite sums with or without numbers . We plan to study those details in subsequent publication.
- It is interesting that (38) happens to be quite similar to
Not often this result appears in this form in the literature, though.
Now, let us see how we evaluate this integral.
Let
Then
We can compute and as follows.
For , recall from Fourier analysis that
It follows that
Finally, we see
Open Question What if we replace by ?
In this article, we encountered many integral representations for dilogarithm, trilogarithm and hence , the Catalan constant and as a reformulation of Boo Rim Choe (1987) [
4], Ewell (1990) [
5] and Williams-Yue (1993) [
6] on the inverse sine function.
As an application, we also proved new Euler sums. Indeed, there are subsequent results on multiple zeta and -values , as Hoffman and Zagier discussed in [
16,
17].
We will write them with more details at another opportunity.
Conflicts of Interest
“The author declares no conflict of interest”.