Integral representations for local dilogarithm and trilogarithm functions

Author(s): Masato Kobayashi1
1 Department of Engineering, Kanagawa University, 3-27-1 Rokkaku-bashi, Yokohama 221-8686, Japan.
Copyright © Masato Kobayashi. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

We show new integral representations for dilogarithm and trilogarithm functions on the unit interval. As a consequence, we also prove (1) new integral representations for Apéry, Catalan constants and Legendre χ functions of order 2, 3, (2) a lower bound for the dilogarithm function on the unit interval, (3) new Euler sums.

Keywords: Apéry constant; Catalan constant; Dilogarithm; Euler sum; Inverse sine function; Riemann zeta function; Trilogarithm; Wallis integral.

1. Introduction

Polylogarithm function

The polylogarithm function Lis(z)=n=1znns=z+z22s+z33s+,s,zC,|z|<1 plays a significant role in many areas of number theory; its origin, the dilogarithm Li2(z), 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 Li3(z) functions and zeta values ζ(2), ζ(3) (Apéry constant), ζ(4) 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 sin1x, they each derived certain infinite sums related to ζ(2) and ζ(3) 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 sin1x=n=0(2n1)!!(2n)!!x2n+12n+1 n=01(2n+1)2=34ζ(2)=π28
Ewell sin1xx=n=0(2n1)!!(2n)!!x2n2n+1 n=01(2n+1)3=78ζ(3)
Williams-Yue (sin1x)2x=12n=1(2n)!!(2n1)!!x2n1n2 π8n=11n3=π8ζ(3)

In this article, we reformulate their ideas introducing Wallis operator and naturally extend their results.

  • We find new integral representations for Li2(t), Li3(t), Legendre χ functions of order 2, 3 and even for Apéry, Catalan constants (Theorems 2, 5, Corollaries 3, 6).
  • We give a lower bound for Li2(t) on the unit interval (Theorem 7).
  • Making use of (sin1x)3 and (sin1x)4, we prove new Euler sums (Theorem 8).

Notation

Throughout this paper, n denotes a nonnegative integer. Let (2n)!!=2n(2n2)42,(2n1)!!=(2n1)(2n3)31. In particular, we understand that (1)!!=0!!=1. Moreover, let wn=(n1)!!n!!. Notice the relation w2nw2n+1=12n+1 as we will see in the sequel.

Remark 1.

  1. The sequence {wn} appears in Wallis integral as 0π/2sinnx={π2wnn even,wnn odd.
  2. It also appears in the literature in the disguise of central binomial coefficients as w2n=(2n1)!!(2n)!!=122n(2nn). See Apéry [7], van der Poorten [8], for example.

Unless otherwise specified, t,u,x,y are real numbers. By sin1x and cos1x, we mean the real inverse sine and cosine functions (arcsinx,arccosx), that is,

y=sin1xx=siny,π2yπ2,y=cos1xx=cosy,0yπ.

Fact 1. (Gradshteyn-Ryzhik [9, p.60, 61])

sin1t=n=0w2nt2n+12n+1,|t|1.
(1)
(sin1t)2=12n=11w2nt2nn2,|t|1.
(2)

Further, sinh1x=log(x+x2+1) (xR) denotes the inverse hyperbolic sine function (some authors write arsinh x,arcsinh x or argsinh x for this one).

2. Dilogarithm function

2.1. Definition

Definition 1. For 0t1, the dilogarithm function is Li2(t)=n=1tnn2.

In particular, Li2(1)=ζ(2)=π26.

It is possible to describe its even part by Li2 itself since

n=1t2n(2n)2=14n=1(t2)nn2=14Li2(t2). Its odd part is called the Legendre χ function of order 2: χ2(t)=n=1t2n1(2n1)2. Here is a fundamental relation of these two parts.

Observation 1. Li2(t)=χ2(t)+14Li2(t2).

Definition 2. Define Ti2(t)=n=1(1)n1(2n1)2t2n1 as a signed analog of χ2(t).

This is also called the inverse tangent integral of order 2 because of the integral representations Ti2(t)=0ttan1xxdx.

2.2. Wallis operator

Let R[[t]] denote the set of power series in t over real coefficients. Set F(t)={fR[[t]]|f(t) is convergent for |t|1}.

Definition 3. For fF(t), define W:F(t)F(t) by Wf(t)=01f(tu)du1u2. Call W 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 W coefficientwise.

Lemma 1. Let f(t)=n=0antnF(t). Then Wf(t)=n=0a2n(π2w2n)t2n+n=0a2n+1w2n+1t2n+1.

Proof. Wf(t)=01f(tu)du1u2=01(n=0a2nt2nu2n+n=0a2n+1t2n+1u2n+1)du1u2=n=0a2nt2n01u2ndu1u2+n=0a2n+1t2n+101u2n+1du1u2=n=0a2n(π2w2n)t2n+n=0a2n+1w2n+1t2n+1.

Observe that W is linear in the sense that W(f+g)=W(f)+W(g) and W(cf)=cW(f) for f,gF(t),cR.

2.3. Main Theorem 1

Lemma 2. All of the following are convergent power series for |t|1.

sin1t=n=0w2nt2n+12n+1.
(3)
12(sin1t)2=n=11w2nt2n(2n)2.
(4)
sin1t+1π(sin1t)2=n=0w2nt2n+12n+1+n=12πw2nt2n(2n)2.
(5)
sinh1t=n=0(1)nw2nt2n+12n+1.
(6)
12(sinh1t)2=n=1(1)n1w2nt2n(2n)2.
(7)

Proof. We already saw (3) and (4) in Introduction. (5) is (3)+2π(4). (6) and (7) follow from (3), (4) and sinh1z=1isin1(iz) (for all zC) [9, p.56].

Theorem 2. For 0t1, all of the following hold;

χ2(t)=01sin1(tu)1u2du.
(8)
14Li2(t2)=1π01(sin1(tu))21u2du.
(9)
Li2(t)=01sin1(tu)+1π(sin1(tu))21u2du.
(10)
Ti2(t)=01sinh1(tu)1u2du.
(11)
π2(14Li2(t2)18Li2(t4))=0112(sinh1tu)21u2du.
(12)

Proof. Note that these are equivalent to the following statements:

W(sin1t)=χ2(t).
(13)
W(12(sin1t)2)=π214Li2(t2).
(14)
W(sin1t+1π(sin1t)2)=Li2(t).
(15)
W(sinh1t)=Ti2(t).
(16)
W(12(sinh1t)2)=π2(14Li2(t2)18Li2(t4)).
(17)
With Lemmas 1 and 2, we can verify (13)-(16) by checking coefficients of those series. For example, W(sin1t)=W(n=0w2nt2n+12n+1)=n=0w2nw2n+1t2n+12n+1=n=0t2n+1(2n+1)2=χ2(t). It remains to show (17). W(12(sinh1t)2)=n=1(1)n1w2n(w2nπ2)t2n(2n)2=π2n=1(1)n1(2n)2t2n=π2(n=11(2n)2t2n2n=11(4n)2t4n)=π2(14Li2(t2)18Li2(t4)).

Corollary 3.

01sin1u1u2du=34ζ(2)=π28.
(18)
2π0112(sin1u)21u2du=14ζ(2)=π224.
(19)
01(sin1u+1π(sin1u)2)du1u2=ζ(2)=π26.
(20)
01sinh1u1u2du=G.
(21)
0112(sinh1u)21u2du=π16ζ(2)=π396.
(22)

Proof. These are χ2(1),14Li2(12),Li2(1),Ti2(1) and π2(14Li2(12)18Li2(14)).

3. Trilogarithm function

3.1. Definition

Definition 4. The trilogarithm function for 0t1 is Li3(t)=n=1tnn3.

Its odd part is the Legendre χ function of order 3: χ3(t)=n=1t2n1(2n1)3. In particular, Li3(1)=ζ(3) and χ3(1)=78ζ(3).

Observation 4. Li3(t)=χ3(t)+18Li3(t2).

Further, a signed analog of χ3(t) is Ti3(t)=n=1(1)n1(2n1)3t2n1.

3.2. Main Theorem 2

Lemma 3.

0tsin1yydy=n=0w2nt2n+1(2n+1)2.
(23)
0t12(sin1y)2ydy=n=11w2nt2n(2n)3.
(24)
0tsin1y+1π(sin1y)2ydy=n=0w2nt2n+1(2n+1)2+n=12πw2nt2n(2n)3.
(25)
0tsinh1yydy=n=0(1)nw2nt2n+1(2n+1)2.
(26)
0t12(sinh1y)2ydy=n=1(1)n1w2nt2n(2n)3.
(27)

Proof. We can derive all of these by integrating (3)-(7) termwise.

As a consequence, we obtain the equalities below (cf. (13)-(17)).
W(0tsin1yydy)=χ3(t).
(28)
W(0t12(sin1y)2ydy)=π218Li3(t2).
(29)
W(0tsin1y+1π(sin1y)2ydy)=Li3(t).
(30)
W(0tsinh1yydy)=Ti3(t).
(31)
W(0t12(sinh1y)2ydy)=n=1(1)n1w2n(w2nπ2)t2n(2n)3=π2n=1(1)n1(2n)3t2n=π2(n=11(2n)3t2n2n=11(4n)3t4n)=π2(18Li3(t2)132Li3(t4)).
(32)
In this way, the five functions above come to possess double integral representations. For example, χ3(t)=01(0tusin1yydy)du1u2. We can indeed simplify such integrals to single ones by exchanging order of integrals.

Theorem 5.

χ3(t)=01sin1(tx)cos1xxdx.
(33)
18Li3(t2)=2π0112(sin1(tx))2cos1xxdx.
(34)
Li3(t)=01(sin1(tx)+1π(sin1(tx))2)cos1xxdx.
(35)
Ti3(t)=01sinh1(tx)cos1xxdx.
(36)
π2(18Li3(t2)132Li3(t4))=0112(sinh1(tx))2cos1xxdx.
(37)

Proof. We give a proof altogether. For t=0, all the equalities hold as 0=0. Suppose 0<t1. Let f(y){sin1y,1π(sin1y)2,sin1y+1π(sin1y)2,sinh1y,12(sinh1y)2}. Then W(0tf(y)ydy)=010tuf(y)ydydu1u2=0ty/t1f(y)y11u2dudy=0tf(y)ycos1ytdy=01f(tx)xcos1xdx.

Corollary 6.

01sin1xcos1xxdx=78ζ(3).
(38)
2π0112(sin1x)2cos1xxdx=18ζ(3).
(39)
01(sin1x+1π(sin1x)2)cos1xxdx=ζ(3).
(40)
01sinh1xcos1xxdx=π332.
(41)
0112(sinh1x)2cos1xxdx=3π64ζ(3).
(42)

Proof. These are χ3(1),18Li3(12),Li3(1),Ti3(1) and π2(18Li3(12)132Li3(14)).

4. Applications

4.1. Inequalities

It is easy to see from the definitions Li2(t)=n=1tnn2 and χ2(t)=n=1t2n1(2n1)2 (0t1) that 0Li2(t)π26and0χ2(t)π28. In fact, we can improve these inequalities a little more. For upper bounds, it is immediate that Li2(t)=n=1tnn2n=1tn2=π26t,χ2(t)=n=1t2n1(2n1)2n=1t(2n1)2=π28t. We next prove nontrivial lower bounds for these functions and also Ti2(t).

Theorem 7. For 0t1,

Li2(t)43π(sin1t)3t.
(43)
χ2(t)(sin1t)22t.
(44)
Ti2(t)(sinh1t)22t.
(45)

Before the proof, we need a lemma. It provides another integral representation of Li2(t) which seems interesting itself.

Lemma 4. For 0t1,

Li2(t)=8tπ01sin1(tx)cos1x1tx2dx.
(46)
(cf.Li3(t)=8π01(sin1(tx))2cos1xxdx,tt in (34).)

Proof. If t=0, then both sides are 0. For 0<t1, RHS=8π0tsin1y1y2cos1ytdy=8π0tsin1y1y2y/t111u2dudy=8π010tusin1y1y2dy11u2du=8πW(0tsin1y1y2dy)=8πW(12(sin1t)2)=8π(π214Li2((t)2))=Li2(t).

Proof of Theorem 7. If t=0, then all of (43)-(45) hold as 00. Suppose t>0. Since sin1 is increasing on [0,1], sin1(tx)sin1(tx) for all 0<t,x1. Then Li2(t)=8tπ01sin1(tx)1tx2cos1xdx8tπ01sin1(tx)1t2x2cos1xdx=8tπ011t(12(sin1tx)2)cos1xdx=8π([12(sin1tx)2cos1x]0100112(sin1tx)211x2dx)4π01(sin1tx)21t2x2dx=4π[13t(sin1tx)3]01=43π(sin1t)3t. Next, we prove (44). Note that sin1(tx)1t2x2sin1(tx)1x2 for 0<t,x<1. Integrate these from 0 to 1 in x so that 01sin1(tx)1t2x2dx01sin1(tx)1x2dx,[(sin1(tx))22t]01χ2(t),(sin1t)22tχ2(t). Quite similarly, for 0<t,x<1, it also holds that sinh1(tx)1+t2x2sinh1(tx)1x2,01sinh1(tx)1+t2x2dx01sinh1(tx)1x2dx=Ti2(t). The left hand side is [sinh1(tx)22t]01=(sinh1t)22t.

4.2. Euler sums

Definition 5. A harmonic number is Hn=k=1n1k. More generally, for m,n1, an (m,n)- harmonic number is Hn(m)=k=1n1km.

In particular, Hn(1)=Hn. Any series involving such numbers is called an Euler sum.

Valean [10, p.292-293] presents truly remarkable Euler sums such as

n=1Hn2n2=174ζ(4),n=1Hn2n3=72ζ(5)ζ(2)ζ(3),n=1Hn2n4=9724ζ(6)2ζ2(3),n=1Hn2n5=6ζ(7)ζ(2)ζ(5)52ζ(3)ζ(4),n=1HnHn(2)n2=ζ(2)ζ(3)+ζ(5). There are many ideas to prove such formulas; Borwein and Bradley [11] gives thirty two proofs for n=1Hn1n2=ζ(3)=8n=1(1)nHn1n2 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 On(2)=H2n1(2)14Hn1(2)=k=12n11k2k=1n11(2k)2=k=0n11(2k+1)2.

Theorem 8.

n=0On(2)(2n+1)2=π4384=1564ζ(4).
(47)
n=1Hn1(2)n2=π4120=34ζ(4).
(48)

For the proof, we make use of less-known Maclaurin series for (sin1t)3 and (sin1t)4; thus we can interpret this result as a natural subsequence of Boo, Ewell and Williams-Yue’s work.

Lemma 5.

(sin1t)3=n=0(6On(2))w2nt2n+12n+1.
(49)
(sin1t)4=12n=1(3Hn1(2))1w2nt2nn2.
(50)
(cf.sin1t=n=0w2nt2n+12n+1,(sin1t)2=12n=11w2nt2nn2).

Proof. First, write (sin1t)3=n=0Ant2n+1, AnR and let an=2n+1w2nAn(n0). It is enough to show that an=6On(2). Since the series (sin1t)3=(t+t36+)3 starts from the t3 term, A0=a0=0. For convenience, set fn(x)=sin2n+1x(2n+1)!. Then fn(x)=sin2nx(2n)!cosx, fn(x)=1(2n)!(2nsin2n1x(1sin2x)sin2n+1x)=fn1(x)(2n+1)2fn(x). Now let x=sin1t (π2xπ2), bn=(2n1)!!. Recall that sin1t=n=0w2nt2n+12n+1. In terms of x, bn,fn(x), this is x=n=0w2nsin2n+1x2n+1=n=0(2n1)!!(2n)!!(2n)!sin2n+1x(2n+1)!=n=0bn2fn(x). Thus, x3=n=0Ansin2n+1x=n=0an(w2nsin2n+1x2n+1)=n=0anbn2fn(x). Differentiate both sides twice in x: 6x=n=0anbn2fn(x)=n=0anbn2(fn1(x)(2n+1)2fn(x))=n=0(an+1bn+12fn(x)anbn2(2n+1)2fn(x)),6n=0bn2fn(x)=n=0(an+1bn+12fn(x)anbn2(2n+1)2fn(x)). Equating coefficients of fn(x) yields 6bn2=an+1bn+12anbn2(2n+1)2,n0. Since bn+1=(2n+1)bn and bn0, we must have an+1an=6(2n+1)2. With a0=0, we now arrive at an=k=0n16(2k+1)2=6On(2), as required.

The proof for (50) proceeds along the same line. Write (sin1t)4=12n=0Cnt2n, CnR and let cn=Cnw2nn2(n1). It is enough to show that cn=3Hn1(2). Since the series (sin1t)4 starts from the t4 term, C1=c1=0. For convenience, set

gn(x)=sin2nx(2n)!. Then gn(x)=sin2n1x(2n1)!cosx, gn(x)=1(2n1)!((2n1)sin2n2x(1sin2x)sin2nx)=gn1(x)(2n)2gn(x). Now let x=sin1t (π2xπ2), dn=2n(n1)!. Recall that (sin1t)2=12n=11w2nt2nn2. In terms of x, dn,gn(x), this is x2=12n=11w2nsin2nxn2=12n=1(2n)!!(2n1)!!(2n)!n2sin2nx(2n)!=12n=1dn2gn(x). Thus, x4=12n=1cn(1w2nsin2nxn2)=12n=1cndn2gn(x). Differentiate both sides twice in x: 12x2=12n=1cndn2gn(x)=12n=1cndn2(gn1(x)(2n)2gn(x))=12n=0cn+1dn+12gn(x)12n=1cndn2(2n)2gn(x)=12n=1cn+1dn+12gn(x)12n=1cndn2(2n)2gn(x)(c1=0)=12n=1(cn+1dn+12cndn2(2n)2)gn(x),122n=1dn2gn(x)=12n=1(cn+1dn+12cndn2(2n)2)gn(x). Equating coefficients of gn(x) yields 122dn2=12(cn+1dn+12cndn2(2n)2),n1. Since dn+1=2ndn and dn0, we must have cn+1cn=12(2n)2. With c1=0, we conclude that cn=k=1n112(2k)2=k=1n13k2=3Hn1(2).

Proof of Theorem 8. Note that W(16(sin1t)3)=n=1O~n(2)w2n2n+1w2n+1t2n+1=n=1O~n(2)(2n+1)2t2n+1. Clearly, t=1 gives the sum for (47). Therefore, W(16(sin1t)3)|t=1=0116(sin1u)3du1u2=[124(sin1u)4]01=π4384. Similarly, we have W(23(sin1t)4)=π2n=1Hn1(2)n2t2n so that W(23(sin1t)4)|t=1=0123(sin1u)4du1u2=[215(sin1u)5]01=π5240. We conclude that n=1Hn1(2)n2=2π(π5240)=π4120.

Remark 3.

  1. (47) is a variation of De Doelder’s formula n=1On(2)n2=π432 [12, p.1196 (13)] and (48) gives another proof of n=1Hn(2)n2=74ζ(4) [10, p.286] because n=1Hn(2)n2=n=1(Hn1(2)n2+1n4)=34ζ(4)+ζ(4)=74ζ(4).
  2. 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 Li2(t),Li3(t). Here, we record several examples. Let ϕ=1+52 be the golden ratio. Observe that ϕ1=512,ϕ2=352. We write log2x for (logx)2. Note that log2(ϕ1)=(log(ϕ1))2=(log(ϕ))2=(log(ϕ))2=log2(ϕ).

Fact 2. ([2]).

Li2(ϕ1)=log2(ϕ)+π210.
(51)
Li2(ϕ2)=log2(ϕ)+π215.
(52)
Li3(ϕ2)=45ζ(3)2π215logϕ+23log3ϕ.
(53)
Li2(12)=π21212log22.
(54)
Li3(12)=78ζ(3)π212log2+16log32.
(55)

Corollary 9.

01sin1(ϕ1u)1u2du=34log2(ϕ)+π212.
(56)
0112(sin1u2)21u2du=π8(π21212log22).
(57)
16π0112(sin1ϕ1x)2cos1xxdx=45ζ(3)2π215logϕ+23log3ϕ.
(58)
0112(sinh1ϕ1/2u)2du1u2=π2(18log2ϕ+π260).
(59)
16π0112(sin1x2)2cos1xxdx=78ζ(3)π212log2+16log32.
(60)

Proof. 01sin1(ϕ1u)1u2du=χ2(ϕ1)=Li2(ϕ1)14Li2(ϕ2)=(log2(ϕ)+π210)14(log2(ϕ)+π215)=34log2(ϕ)+π212. W(12(sin1t)2)|t=1/2=π8Li2(12)=π8(π21212log22). (34) for t=ϕ1 with (53) gives (58). W(12(sinh1t)2)|t=ϕ1/2=π2(14Li2(ϕ1)18Li2(ϕ2))=π2(18log2ϕ+π260). Finally, (34) for t=1/2 with (55) gives (60).

5. Concluding remarks

Here, we record several remarks for our future research.
  1. For 0α1, define a generalized Wallis operator Wαf(t)=0αf(tu)du1u2 so that we can deal with more general integrals. Study Wα, particularly for α=1/2,2/2,3/2.
  2. Can we show any inequality for Li3(t),χ3(t) and Ti3(t) in a similar way?
  3. Discuss (sinh1t)3, (sinh1t)4 and related Euler sums.
  4. Wolfram alpha [15] says that 01(sin1x)3xdx=0π/2u3cotudu=π38log2916πζ(3),01(sin1x)4xdx=0π/2u4cotudu=132(18π2ζ(3)+93ζ(5)+2π4log2). It should be possible to describe such integrals as certain infinite sums with or without numbers w2n. We plan to study those details in subsequent publication.
  5. It is interesting that (38) happens to be quite similar to 01tan1xcot1xxdx=78ζ(3). Not often this result appears in this form in the literature, though. Now, let us see how we evaluate this integral. Let I=01tan1xcot1xxdx,I1=01tan1xxdx,I2=01(tan1x)2xdx. Then I=01tan1xcot1xxdx=01tan1x(π2tan1x)xdx=π2I1I2. We can compute I1 and I2 as follows. I1=01tan1xxdx=01n=0(1)n2n+1x2ndx=n=0(1)n2n+101x2ndx=n=0(1)n(2n+1)2=G. For I2, recall from Fourier analysis that log(tany2)=2n=012n+1cos(2n+1)y,0<y<π. It follows that I2=01(tan1x)2xdx=[y=2tan1x]140π/2y2sinydy=14([y2log(tany2)]0π/20π/22ylog(tany2)dy)=120π/2y(2n=012n+1cos(2n+1)y)dy=n=012n+10π/2ycos(2n+1)ydy=n=012n+1([ysin(2n+1)y2n+1]0π/20π/2sin(2n+1)y2n+1dy)=n=0(π2(1)n(2n+1)21(2n+1)3)=πG278ζ(3). Finally, we see I=πG2(πG278ζ(3))=78ζ(3).

Open Question What if we replace tan1 by tanh1?

In this article, we encountered many integral representations for dilogarithm, trilogarithm and hence ζ(2), the Catalan constant G and ζ(3) 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 t-values ζ(3,2,,2), t(3,2,,2) 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”.

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