Explicit Evaluations of Sums of Sequence Tails Ce Xu∗ Xiaolian Zhou† School of Mathematical Sciences, Xiamen University 7 Xiamen 361005,P.R. China 1 0 2 n Abstract In this paper, we use Abel’s summation formula to evaluate several quadratic sums a of the form: J N 4 F (A,B;x) := (A−A )(B−B )xn, x ∈[−1,1], N n n ] nX=1 T where the sequences A ,B are defined by the sums n n N n n . th An = ak,Bn = bk, ak,bk = o(n−p),ℜ(p) > 1 a k=1 k=1 m X X [ and A = lim An,B = lim Bn,F (A,B;x) = lim Fn(A,B;x). We give an explicit formula n→∞ n→∞ n→∞ 1 of Fn(A,B;x). Furthermore, we also evaluate several other series involving multiple zeta star v values. 5 2 Keywords Sequence; harmonic numbers; Abel’s summation formula; Riemann zeta function; 7 multiple zeta star values, tails. 3 0 AMS Subject Classifications (2010): 11M06; 11M32. . 1 0 7 1 Introduction 1 : v In recent years there has been significant interest in investigating sums of products of Riemann i X zeta tails and multiple zeta values. The subject of this paper is studying quadratic and cubic r sums a ∞ ∞ F (A,B;x) = (A−A )(B −B )xn,F (A,B,ζ(r))= (A−A )(B −B )(ζ(r)−ζ (r)), n n n n n n=1 n=1 X X where n n 1 1 H = ζ (1) = , ζ (r)= , r,n ∈ N= {1,2,3···} n n j n jr j=1 j=1 X X are classical harmonic number and generalized harmonic numbers or partial sums of the series ∞ 1 ζ(s)= , ℜ(s)> 1 js j=1 X ∗Corresponding author. Email: [email protected] †Email: [email protected](X. ZHOU) 1 defining the Riemann zeta function, respectively, where the empty sum ζ0(r) is conventionally understood to be zero. In recent papers [9,10], O. Furdui, A. Sˆıntˇamaˇrian and C. V˘alean prove some results on sums of products of the tails of the Riemann zeta function, i.e., ∞ 1 ζ(s)−ζ (s)= . n js j=n+1 X In [9], O. Furdui and A. Sˆıntˇamaˇrian shown that quadratic series ∞ F(ζ(s),ζ(s)) = (ζ(s)−ζ (s))2, s ∈ N/{1} n n=1 X can be expressed as a rational linear combination of zeta function values, binomial coefficients and products of two zeta function values. In [10], O. Furdui and C. V˘alean proven that the series ∞ F(ζ(s),ζ(s+1)) = (ζ(s)−ζ (s))(ζ(s+1)−ζ (s+1)), s∈ N/{1} n n n=1 X can be expressed in terms of Riemann zeta function values and a special integral involving a polylogarithm function. The polylogarithm function defined as follows ∞ xn Li (x) = ,ℜ(p)> 1, |x|≤ 1. p np n=1 X Furthermore, in [12], M. E. Hoffman shown a simple general result for the quadratic sum ∞ F(ζ(p),ζ(q)) = (ζ(p)−ζ (p))(ζ(q)−ζ (q)), p,q ∈ N/{1} n n n=1 X in terms of the multiple zeta values ζ(s1,s2,...,sm) defined by 1 ζ(s1,s2,...,sm) = ks1···ksm, k1>··X·>km≥1 1 m wherefor convergence s1+s2+···+sj > j for j = 1,2,...,m. Moreover, Hoffman also provided an explicit evaluation of sums of products of three or more Riemann zeta tails in a closed form in terms of multiple zeta values (see Theorem 4.3 in the reference [12]). The multiple zeta value ζ(s1,s2,...,sm)issaid tohavedepthmandweight s1+...+sm. Multiplezeta valuesofgeneral depthwereintroducedindependentlyby Hoffman [11] andD.Zagier [19], butfordepthtwo they were already studied by Euler. In [5], Borwein and Girgensohn gave some evaluation multiple zeta values of depth three. In this paper, we discuss the analytic representations of F(A,B;x) and F(A,B,ζ(r)). Using certain integral representations, we can prove that the cubic series ∞ F (ζ(p),ζ(q),ζ(r)) = (ζ(p)−ζ (p))(ζ(q)−ζ (q))(ζ(r)−ζ (r)), p,q,r ∈ N/{1} n n n n=1 X can be expressed in terms of multiple zeta star values defined by 1 ζ⋆(s1,s2,...,sm)= ks1···ksm, s1 ∈ N/{1},si ∈ N,(i = 2,3,...). k1≥··X·≥km≥1 1 m 2 ∞ ζ (p) Obviously, ζ⋆(m,p) = n . Furthermore,wealsogive someevaluation ofsumsofproducts nm n=1 X of alternating Riemann Zeta Tails. For example, ∞ (ζ(m)−ζ (m)) ζ¯(p)−ζ¯ (p) n n n=1 X (cid:0) (cid:1) ∞ ζ (m−1) ζ¯ (p−1) = n (−1)n−1+ n −ζ(m)ζ¯(p)−ζ¯(m+p−1), np nm n=1(cid:26) (cid:27) X whereζ¯(p)andζ¯ (p)standforthealternatingRiemannZetafunctionandalternating harmonic n numbers defined by ∞ k−1 n k−1 (−1) (−1) ζ¯(p)= ,ζ¯ (p)= ,n ∈N,ℜ(p) ≥ 1. kp n kp k=1 k=1 X X ∞ ζ (p) ∞ ζ¯ (p) ∞ ζ (p) ∞ ζ¯ (p) n n n−1 n n−1 n The evaluation of linear sums , (−1) , (−1) and in nm nm nm nm n=1 n=1 n=1 n=1 X X X X terms of values of Riemann zeta function (or alternating Riemann Zeta function) and polylog- arithm function at positive integers is known when (p,m) = (1,3),(2,2) or p+m is odd (see [2,3,7,14,17,18]). For example, ∞ ζ¯ (1) 7 5 n = ζ(3)ln2− ζ(4), n3 4 16 n=1 X ∞ H 1 11 1 1 7 n n−1 2 4 n3 (−1) = −2Li4 2 + 4 ζ(4)+ 2ζ(2)ln 2− 12ln 2− 4ζ(3)ln2, n=1 (cid:18) (cid:19) X ∞ ζ¯ (1) 3 1 1 1 n n−1 2 4 n3 (−1) = 2ζ(4)+ 2ζ(2)ln 2− 12ln 2−2Li4 2 , n=1 (cid:18) (cid:19) X ∞ ζ (2) 51 1 7 1 n n−1 2 4 n2 (−1) = −16ζ(4)+4Li4 2 + 2ζ(3)ln2−ζ(2)ln (2)+ 6ln 2. n=1 (cid:18) (cid:19) X Furthermore, we consider the nested sum ∞ yn n xk , x,y ∈ [−1,1), m,p ∈ N. nm kp ! n=1 k=1 X X By takingthesumover complementary pairsofsummationindices, weobtain asimplereflection formula ∞ yn n xk ∞ xn n yk nm kp + np km = Lip(x)Lim(y)+Lip+m(xy). ! ! n=1 k=1 n=1 k=1 X X X X Therefore, we can deduce that ∞ ζ¯ (2) 13 n n−1 (−1) = ζ(4), n2 16 n=1 X 3 ∞ ζ (3) 19 3 n n−1 (−1) = ζ(4)− ζ(3)ln2, n 16 4 n=1 X ∞ ζ¯ (2) 85 1 1 7 n 2 4 n2 = 16ζ(4)−4Li4 2 +ζ(2)ln 2− 6ln 2− 2ζ(3)ln2, n=1 (cid:18) (cid:19) X ∞ ζ¯ (3) 1 3 1 1 1 n n−1 4 2 (−1) =2Li4 + ζ(3)ln2+ ln 2− ζ(4)− ζ(2)ln 2. n 2 4 12 2 2 n=1 (cid:18) (cid:19) X Some results for sums of harmonic numbers or alternating harmonic numbers may be seen in the works of [2,3,7,13-18] and references therein. The main result of this paper is the following theorems. Theorem 1.1 If a ,b = o(n−p),ℜ(p) > 1 and x ∈ [−1,1], then k k n (1−x)F (A,B;x) = (1−x) (A−A )(B −B )xk n k k k=1 X n n = ABx−(A−A )(B −B )xn+1− a xk (B −B )− b xk (A−A ) n n k n k n ! ! k=1 k=1 X X n k n k n − a xi b − b xi a + a b xk. (1.1) i k i k k k ! ! k=1 i=1 k=1 i=1 k=1 X X X X X Theorem 1.2 If a ,b = o(n−p),ℜ(p) > 1 and r ∈ N/{1}, then k k ∞ F (A,B,ζ(r))= (A−A )(B −B )(ζ(r)−ζ (r)) k k k k=1 X ∞ n ∞ n ∞ =ζ(r) ka b + kb a − na b −AB k n k n n n ( ! ! ) n=1 k=1 n=1 k=1 n=1 X X X X X ∞ n ∞ n − a (kζ (r)−ζ (r−1)) b − b (kζ (r)−ζ (r−1)) a k k k n k k k n ! ! n=1 k=1 n=1 k=1 X X X X ∞ + a b (nζ (r)−ζ (r−1)). (1.2) n n n n n=1 X In section 2, we will use Abels summation formula to prove Theorem 1.1 and 1.2. The Abels summationbypartsformulastatesthatif(an)n≥1 and(bn)n≥1 aretwosequencesofrealnumbers n n n and An = ak, then akbk = Anbn+1+ Ak(bk −bk+1). k=1 k=1 k=1 X X X 2 Proof of Theorem 1.1 and Theorem 1.2 We now prove our Theorems. Firstly, we are ready to prove Theorem 1.1. Proof. From the definition F (A,B;x), we have n n F (A,B;x) = (A−A )(B−B )xk n k k k=1 X 4 n−1 = (A−a1)(B −b1)x+ (A−Ak+1)(B −Bk+1)xk+1 k=1 X n−1 = (A−a1)(B −b1)x+ (A−Ak −ak+1)(B −Bk −bk+1)xk+1 k=1 X n−1 = (A−a1)(B −b1)x+ (A−Ak)(B −Bk)xk+1 k=1 X n−1 n−1 − {ak+1(B−Bk)+bk+1(A−Ak)}xk+1+ ak+1bk+1xk+1 k=1 k=1 X X n = (A−a1)(B −b1)x+ (A−Ak)(B −Bk)xk+1−(A−An)(B−Bn)xn+1 k=1 X n n + akbkxk −a1b1x− {ak(B−Bk +bk)+bk(A−Ak +ak)}xk +a1B +b1A k=1 k=1 X X = xF (A,B;x)+ABx−(A−A )(B −B )xn+1 n n n n n − {a (B −B )+b (A−A )}xk − a b xk. (2.1) k k k k k k k=1 k=1 X X By Abel’s summation formula, we can deduce that n n n k n {a (B −B )}xk = a xk (B−B )+ a xi b − a b xk, (2.2) k k k n i k k k ! ! k=1 k=1 k=1 i=1 k=1 X X X X X n n n k n {b (A−A )}xk = b xk (A−A )+ b xi a − a b xk. (2.3) k k k n i k k k ! ! k=1 k=1 k=1 i=1 k=1 X X X X X Substituting (2.2) (2.3) into (2.1) respectively, we can obtain (1.1). We have completed the lnr−1x proof of Theorem 1.1. Now, we are ready to prove Theorem 1.2. Multiplying (1.2) by 2 (1−x) and integrating over the interval (0,1), and using the following elementary integral identities 1 xklnr−1x dx = (−1)r−1(r−1)!(ζ(r)−ζ (r)), r ∈ N/{1}, k 1−x Z0 1 xklnr−1x dx = (−1)r−1(r−1)!(ζ(r−1)−ζ (r−1)−kζ(r)+kζ (r)),r ∈ N/{1,2}, 2 k k (1−x) Z0 by a direct calculation, we obtain F (A,B,ζ(r))=AB(ζ(r−1)−ζ(r)) ∞ + a b (ζ(r−1)−ζ (r−1)−kζ(r)+kζ (r)) k k k k k=1 X 5 ∞ n − a (ζ(r−1)−ζ (r−1)−kζ(r)+kζ (r)) b k k k n ! n=1 k=1 X X ∞ n − b (ζ(r−1)−ζ (r−1)−kζ(r)+kζ (r)) a . (2.4) k k k n ! n=1 k=1 X X Taking x = 1 in (1.1), we get n n n A b + B a = a b +A B . (2.5) k k k k k k n n k=1 k=1 k=1 X X X Letting n → ∞, then have ∞ ∞ ∞ A b + B a = a b +AB. (2.6) k k k k k k k=1 k=1 k=1 X X X Substituting (2.6) into (2.4), we know that when r ∈ N/{1,2}, the equality (1.2) is holds. lnx Similarly, multiplying (1.2) by and integrating over the interval (0,t), then letting 2 (1−x) t → 1, we can prove that if r = 2, the formula (1.2) is also true. The proof of Theorem 1.2 is finished. 3 Some results In this section, we will give some closed form of sums of sequences tails. Firstly, multiplying (1−x)−1 in both sides of (1.1), then letting x → 1 and using L. Hopital’s rule, we deduce that n (1−x)F (A,B;x) n (A−A )(B −B )= F (A,B;1) = lim k k n x→1 1−x k=1 X n k n k n n = ia b + ib a + ka (B −B )+ kb (A−A ) i k i k k n k n ! ! ! ! k=1 i=1 k=1 i=1 k=1 k=1 X X X X X X n +(n+1)(A−A )(B −B )− ka b −AB. (3.1) n n k k ! k=1 X Letting n → ∞ in (3.1), we have ∞ ∞ n ∞ n ∞ (A−A )(B −B )= ka b + kb a − na b −AB. (3.2) n n k n k n n n ! ! n=1 n=1 k=1 n=1 k=1 n=1 X X X X X X n−1 n−1 n−1 1 1 (−1) (−1) 1 (−1) Putting (a ,b ) = , , , , , in (3.2), we can obtain n n nm np nm np nm np ! ! (cid:18) (cid:19) the following identities ∞ (ζ(m)−ζ (m))(ζ(p)−ζ (p)) n n n=1 X 6 = ζ⋆(p,m−1)+ζ⋆(m,p−1)−ζ(m+p−1)−ζ(m)ζ(p), (3.3) ∞ ζ¯(m)−ζ¯ (m) ζ¯(p)−ζ¯ (p) n n n=1 X(cid:0) (cid:1)(cid:0) (cid:1) ∞ ζ¯ (p−1) ∞ ζ¯ (m−1) = n (−1)n−1+ n (−1)n−1−ζ(m+p−1)−ζ¯(m)ζ¯(p), (3.4) nm np n=1 n=1 X X ∞ (ζ(m)−ζ (m)) ζ¯(p)−ζ¯ (p) n n n=1 X (cid:0) (cid:1) ∞ ζ (m−1) ζ¯ (p−1) = n (−1)n−1+ n −ζ(m)ζ¯(p)−ζ¯(m+p−1). (3.5) np nm n=1(cid:26) (cid:27) X 1 1 Similarly, taking (a ,b ) = , in (1.2), we arrive at the conclusion that n n nm np (cid:18) (cid:19) ∞ F (ζ(m),ζ(p),ζ(r)) = (ζ(m)−ζ (m))(ζ(p)−ζ (p))(ζ(r)−ζ (r)) n n n n=1 X =ζ(r){ζ⋆(p,m−1)+ζ⋆(m,p−1)−ζ(m+p−1)−ζ(m)ζ(p)} +{ζ⋆(m+p−1,r)−ζ⋆(m+p,r−1)} −{ζ⋆(p,m−1,r)−ζ⋆(p,m,r−1)} −{ζ⋆(m,p−1,r)−ζ⋆(m,p,r−1)}. (3.6) lnr−1x In (1.1), letting n → ∞, then multiplying it by and integrating over the interval (0,1), x(1−x) we conclude that ∞ ∞ n ∞ n (A−A )(B −B ) n n nr = akζk−1(r) bn+ bkζk−1(r) an ! ! n=1 n=1 k=1 n=1 k=1 X X X X X ∞ − anbnζn−1(r), r ∈ N/{1}. (3.7) n=1 X In fact, it is easily shown that the formula (3.7) is also true when r = 1. 1 1 Taking (a ,b )= , in (3.7), we get n n nm np (cid:18) (cid:19) ∞ (ζ(m)−ζ (m))(ζ(p)−ζ (p)) n n nr n=1 X = ζ(m+p+r)+ζ⋆(m,p,r)+ζ⋆(p,m,r) −ζ⋆(m+p,r)−ζ⋆(m,p+r)−ζ⋆(p,m+r),m,p,r ∈ N. (3.8) We define the multiple harmonic star number ζn⋆(s1,s2,...,sm) by 1 ζn⋆(s1,s2,...,sm) := ks1···ksm,si ∈ N, (i= 1,2,...,m). 1≤km≤X···≤k1≤n 1 m 7 From the definition multiple harmonic star number, we can deduce the following relation ∞ ζ (p)ζ (r) ζ⋆(p,m,r)+ n n = ζ⋆(p+m,r)+ζ(p)ζ⋆(m,r), p,m ∈ N/{1},r ∈ N. (3.9) nm n=1 X Substituting (3.9) into (3.8) respectively, we obtain the symmetry identity ∞ ζ (m)ζ (p) ζ (p)ζ (r) ζ (m)ζ (r) n n n n n n + + nr nm np n=1(cid:26) (cid:27) X = ζ⋆(p+m,r)+ζ⋆(p+r,m)+ζ⋆(m+r,p) +ζ(p)ζ(m)ζ(r)−ζ(p+m+r), p,m,r ∈ N/{1}. (3.10) ∞ ζ (m)ζ (p) n n From [7,17], we know that the quadratic sums can be evaluated in terms of nr n=1 zeta values and double zeta star values ζ⋆(s1,s2)Xin the following cases: m = p =r or m+p+r even (m,p,r ∈N/{1}) and m = 1,p+r ≤ 8. From example ∞ H ζ (3) 83 1 11 n n = ζ(7)+ ζ(3)ζ(4)− ζ(2)ζ(5), n3 8 4 2 n=1 X ∞ H ζ (2) 343 5 3 n n = − ζ(8)+12ζ(3)ζ(5)− ζ(2)ζ2(3)− ζ⋆(6,2), n5 48 2 4 n=1 X ∞ ζ2(2) 457 n = 11ζ⋆(6,2)+ ζ(8)+6ζ(2)ζ2(3)−40ζ(3)ζ(5). n4 18 n=1 X ∞ ζ (2) where ζ⋆(6,2) = n . n6 n=1 X Similarly, from (1.1), we have the following results ∞ n−1 (ζ(m)−ζ (m))(ζ(p)−ζ (p))(−1) n n n=1 X 1 ∞ ζ¯ (p) ∞ ζ¯ (m) = ζ(m)ζ(p)+ζ¯(m+p)− n − n , (3.11) 2 nm np ( ) n=1 n=1 X X ∞ (ζ⋆(p+1,p)−ζ⋆(p+1,p))(ζ⋆(q+1,q)−ζ⋆(q+1,q)) n n n=1 X 1 ∞ ζ2(p)ζ (q)+ζ (2p)ζ (q) ζ2(q)ζ (p)+ζ (2q)ζ (p) = n n n n + n n n n 2 nq+1 np+1 n=1(cid:26) (cid:27) X ∞ ∞ ∞ ζ (p)ζ (q) ζ (p) ζ (q) n n n n − − , (3.12) np+q+1 np+1 nq+1 ! ! n=1 n=1 n=1 X X X ∞ (ζ⋆(p+1,p)−ζ⋆(p+1,p))(ζ⋆(p+2,p)−ζ⋆(p+2,p)) n n n=1 X 2 1 ∞ ζ3(p)+ζ (p)ζ (2p) ζ2(p) 1 ∞ ζ (p) = n n n − n + n 2 np+2 n2p+2 2 np+1 ! n=1(cid:26) (cid:27) n=1 X X 8 ∞ ∞ ζ (p) ζ (p) n n − . (3.13) np+1 np+2 ! ! n=1 n=1 X X Next, we give a Theorem. Theorem 3.1 Let m,p ≥ 2 be integers, then have ∞ ∞ ∞ ζ(m)ζ (p)−ζ(p)ζ (m) H H n n n n = ζ(p) −ζ(m) +ζ(m)ζ(p+1)−ζ(p)ζ(m+1). n nm np n=1 n=1 n=1 X X X (3.14) Proof. First, we construct the following power function ∞ y = {H ζ (m)−ζ (m+1)}xn−1, x ∈(−1,1). n n n n=1 X By the definition of harmonic numbers, we can deduce that ∞ 1 1 1 y = H + ζ (m)+ − ζ (m+1)+ xn n n+1 n (n+1)m n (n+1)m+1 n=1(cid:26)(cid:18) (cid:19)(cid:18) (cid:19) (cid:18) (cid:19)(cid:27) X ∞ H ζ (m) = H ζ (m)−ζ (m+1)+ n + n xn. (3.15) n n n m (n+1) n+1 n=1(cid:26) (cid:27) X Therefore, from (3.15), we obtain the following relation ∞ ∞ H ζ (m) xn {H ζ (m)−ζ (m+1)}xn−1 = n + n . (3.16) n n n m (n+1) n+1 1−x n=1 n=1(cid:26) (cid:27) X X Then multiplying (3.16) by lnp−1x and integrating over (0,1), we have ∞ ∞ H ζ (m)−ζ (m+1) H ζ (m) n n n n n = + {ζ(p)−ζ (p)}. (3.17) np (n+1)m n+1 n n=1 n=1(cid:26) (cid:27) X X By a direct calculation, we obtain that ∞ ∞ ∞ ∞ H ζ (m) H ζ (p) H H ζ (m) n n n n n n n + = ζ(p) + + np nm nm np+m np+1 n=1(cid:26) (cid:27) n=1 n=1 n=1 X X X X ∞ ∞ ζ (m+1) ζ (m) n n − + {ζ(p)−ζ (p)}. (3.18) np n n n=1 n=1 X X Change m,p to p,m, we get ∞ ∞ ∞ ∞ H ζ (p) H ζ (m) H H ζ (p) n n n n n n n + = ζ(m) + + nm np np nm+p nm+1 n=1(cid:26) (cid:27) n=1 n=1 n=1 X X X X ∞ ∞ ζ (p+1) ζ (p) n n − + {ζ(m)−ζ (m)}. (3.19) nm n n n=1 n=1 X X 9 Combining (3.18) and (3.19), we can obtain (3.14). (cid:3) We note that ∞ ζ(m)ζ(p)−ζ (m)ζ (p) n n n n=1 X ∞ ∞ ζ(m)−ζ (m) ζ (m)(ζ(p)−ζ (p)) = ζ(p) n + n n , m,p ∈ N/{1}. (3.20) n n n=1 n=1 X X In [10], O. Furdui and C. Valean gave the following formula ∞ ζ(m)−ζ (m) n = ζ⋆(m,1)−ζ(m+1). (3.21) n n=1 X Hence, combining (3.18), (3.20) and (3.21), we have the following corollary. Corollary 3.2 For positive integers m > 1,p > 1, then we have ∞ ζ(m)ζ(p)−ζ (m)ζ (p) n n n n=1 X ∞ H ζ (m) H ζ (p) = n n + n n +ζ⋆(p,m+1) np nm n=1(cid:26) (cid:27) X −ζ(p)ζ(m+1)−ζ⋆(p+m,1)−ζ⋆(p+1,m). (3.22) Similarly, noting that ∞ ζ (m)ζ(p)ζ(q)−ζ(m)ζ (p)ζ (q) n n n n n=1 X ∞ ∞ ζ(m)ζ(p)−ζ (m)ζ (p) ζ(m)−ζ (m) = ζ(m) n n −ζ(p)ζ(q) n , m,p,q ∈ N/{1}. n n n=1 n=1 X X Therefore, we also obtain the following corollary. Corollary 3.3 For positive integers m,p,q > 1, then we have ∞ ζ (m)ζ(p)ζ(q)−ζ(m)ζ (p)ζ (q) n n n n n=1 X ∞ H ζ (q) H ζ (p) n n + n n +ζ⋆(p,q+1) = ζ(m) np nq n=1(cid:18) (cid:19) −Xζ⋆(p+q,1)−ζ⋆(p+1,q)−ζ(p)ζ(q+1) −ζ(p)ζ(q){ζ⋆(m,1)−ζ(m+1)}. (3.23) Furthermore, we consider the following power series function ∞ y = {H ζ (p)ζ (q)−ζ (p+q+1)}xn−1, x ∈ (−1,1), n n n n n=1 X proceeding in a similar fashion to evaluation of the Theorem 3.1, we can give the following Theorem. 10