Acta Univ. Sapientiae, Mathematica, 8, 2 (2016) 282{297 DOI: 10.1515/ausm-2016-0019 Several identities involving the falling and rising factorials and the Cauchy, Lah, and Stirling numbers Feng Qi Institute of Mathematics, Henan Polytechnic University, China College of Mathematics, Inner Mongolia University for Nationalities, China Department of Mathematics, College of Science, Tianjin Polytechnic University, China email: [email protected] Xiao-Ting Shi Fang-Fang Liu Department of Mathematics, Department of Mathematics, College of Science, College of Science, Tianjin Polytechnic University, China Tianjin Polytechnic University, China email: [email protected] email: [email protected] Abstract. In the paper, the authors find several identities, including a new recurrence relation for the Stirling numbers of the first kind, in- volving the falling and rising factorials and the Cauchy, Lah, and Stirling numbers. 1 Notation and main results It is known that, for x 2 R, the quantities x(x - 1) ··· (x - n + 1); n ≥ 1 n-1 Γ(x + 1) hxin = = (x - `) = 1; n = 0 Γ(x - n + 1) `=0 Y 2010 Mathematics Subject Classification: 11B73, 11B83 Key words and phrases: identity, Cauchy number, Lah number, Stirling number, falling factorial, rising factorial, Fa`adi Bruno formula 282 Identities involving Cauchy, Lah, and Stirling numbers 283 and x(x + 1) ··· (x + n - 1); n ≥ 1 n-1 Γ(x + n) (x)n = = (x + `) = 1; n = 0 Γ(x) `=0 Y are respectively called the falling and rising factorials, where n!nz Γ(z) = lim n ; z 2 n f0; -1; -2; : : : g n C k=0(z + k) is the classical gamma!1 function,Q see [1, p. 255, 6.1.2]. For removable singulari- Γ(x+m) ties of the ratio Γ(x+n) for x 2 R and m; n 2 Z, please read [23, Theorem 1.1] and closely related references therein. According to [4, pp. 293{294], there are two kinds of Cauchy numbers which may be defined respectively by 1 1 Cn = hxin d x and cn = (x)n d x: (1) Z0 Z0 The Cauchy numbers Cn and cn play important roles in some fields, such as approximate integrals, the Laplace summation formula, and difference- differential equations, and are also related to some famous numbers such as the Stirling, Bernoulli, and harmonic numbers. For recent conclusions on the Cauchy numbers, please read the papers [17, 18, 21, 30]. It is known that the coefficients expressing rising factorials (x)n in terms of falling factorials hxik are called the Lah numbers, denoted by L(n; k). Precisely speaking, n n n-k (x)n = L(n; k)hxik and hxin = (-1) L(n; k)(x)k: (2) k=1 k=1 X X They can be computed by n - 1n! L(n; k) = k - 1 k! and have an interesting meaning in combinatorics: they count the number of ways a set of n elements can be partitioned into k nonempty linearly ordered subsets. For more and recent results on the Lah numbers L(n; k), please refer to [13, 15, 16]. 284 F. Qi, X.-T. Shi, F. -F. Liu The Stirling numbers of the first kind s(n; k) may be generated by n n k n-k k hxin = s(n; k)x and (x)n = (-1) s(n; k)x : (3) k=0 k=0 X X The combinatorial meaning of the unsigned Stirling numbers of the first kind (-1)n-ks(n; k) can be interpreted as the number of permutations of f1; 2; : : : ; ng with k cycles. Recently there are some new results on the Stirling numbers of the first kind s(n; k) obtained in [17, 20, 21, 22]. An infinitely differentiable function f is said to be completely monotonic on an interval I if it satisfies (-1)nf(n)(x) ≥ 0 for x 2 I and n ≥ 0. See [38, Definition 1.3] and [40, Chapter XII]. An infinitely differentiable function f : I ⊆ (- ; ) [0; ) is called a Bernstein function on I if its derivative f0(t) is completely monotonic on I. See [38, Definition 3.1]. The1 class1 of! completely1 monotonic functions may be characterized by [40, Theorem 12b] which reads that a necessary and sufficient condition that f(x) -xt should be completely monotonic for 0 < x < is that f(x) = 0 e d α(t), where α(t) is non-decreasing and the integral converges for 0 < x < . The R1 Bernstein functions on (0; ) can be characterized1 by the assertion that a function f :(0; ) R is a Bernstein function if and only if it admits1 the representation 1 1 !f(x) = a + bx + 1 - e-xt d µ(t); (4) Z01 where a; b ≥ 0 and µ is a Radon measure on (0; ) satisfying 0 minf1; tg d µ(t) < : See [38, Theorem 3.2]. The triplet (a; b; µ) determines f uniquely and R1 vice versa. The representing measure µ and the1 characteristic triplet (a; b; µ) from1 the expression (4) are often called the L´evymeasure and the L´evytriplet of the Bernstein function f. The formula (4) is called the L´evy-Khintchine representation of f. It was obtained inductively in [32, Lemma 2.1] that the derivatives of the functions 1α h (t) = 1 + ; t > 0; α 2 (-1; 1) α t and h (t) h (t) H (t) = α - α-1 α α α - 1 may be computed by (-1)i 1α i-1 ii - 1 h(i)(t) = 1 + k! (α) tk (5) α ti(1 + t)i t k k i-k k=0 X Identities involving Cauchy, Lah, and Stirling numbers 285 and (-1)i 1α i-1 i + 1i - 1 H(i)(t) = 1 + k! (α) tk; α ti(1 + t)i+1 t k k i-k k=0 X for i 2 N. Consequently, 1. if α 2 (0; 1), the function hα(t) is completely monotonic on (0; ); 2. if α 2 (-1; 0), the function h (t) is a Bernstein function on (0; ); α 1 3. if α 2 (0; 1), the function H (t) is completely monotonic on (0; ). α 1 With the help of [32, Lemma 2.1], it was derived in [32, Theorem 1.1] that the 1 weighted geometric mean λ 1-λ Gx;y;λ(t) = (x + t) (y + t) is a Bernstein function of t > - minfx; yg, where λ 2 (0; 1) and x; y 2 R with x 6= y. For more and detailed information on this topic, please refer to [2, 12, 22, 26, 32, 33, 34, 35, 36, 42] and closely related references therein. In combinatorics, the Bell polynomials of the second kind (also called the partial Bell polynomials) Bn;k(x1; x2; : : : ; xn-k+1) are defined by n-k+1 n! x `i B (x ; x ; : : : ; x ) = i n;k 1 2 n-k+1 n-k+1 i! 1≤i≤n i=1 `i! i=1 `i2Xf0g[N Y n i=1 i`i=n Q n `i=k P i=1 P for n ≥ k ≥ 0, see [4, p. 134, Theorem A]. The Fa`adi Bruno formula may be described in terms of the Bell polynomials of the second kind Bn;k by dn n f ◦ g(x) = f(k)(g(x))B g0(x); g00(x); : : : ; g(n-k+1)(x); (6) d xn n;k k=0 X see [4, p. 139, Theorem C]. The aims of this paper are, by virtue of the famous Fa`adi Bruno formula (6), to find a new form for derivatives of the function hα(t), and then, by comparing this new form with (5), to derive some identities involving the falling and rising factorials and the Cauchy, Lah, and Stirling numbers. Our main results may be summarized up as the following theorem. 286 F. Qi, X.-T. Shi, F. -F. Liu Theorem 1 For i 2 N and α 2 R, i-1 " i-k # (-1)i 1α i - m h(i)(t) = 1 + L(i; m)hαi tk: (7) α ti(1 + t)i t k m k=0 m=1 X X Consequently, the identities 1 n n + k - m (α) = L(n + k; m)hαi ; (8) n n+kn+k-1 k m k! k k m=1 X 1 n n + k - m c = L(n + k; m)C ; (9) n n+kn+k-1 k m k! k k m=1 X 1 n n n + k - m c = (-1)m-` L(n + k; m)L(m; `)c ; n n+kn+k-1 k ` k! k k `=1 m=` X X (10) and (-1)n-` n n + k - m s(n; `) = L(n + k; m)s(m; `) (11) n+kn+k-1 k k! k k m=` X hold for all k; ` ≥ 0 and n 2 N. In next section, we will give a proof of Theorem1. In the final section, we will list some remarks for explaining and interpreting the significance of identities obtained in Theorem1. 2 Proof of Theorem1 Now we are in a position to prove formulas or identities listed in Theorem1. The Bell polynomials of the second kind Bn;k(x1; x2; : : : ; xn-k+1) satisfy 2 n-k+1 k n Bn;k abx1; ab x2; : : : ; ab xn-k+1 = a b Bn;k(x1; x2; : : : ; xn-k+1) (12) and Bn;k(1!; 2!; 3!;:::; (n - k + 1)!) = L(n; k); (13) see [4, p. 135], where a and b are any complex numbers. Identities involving Cauchy, Lah, and Stirling numbers 287 α 1 Taking in (6) f(u) = (1 + u) and u = g(t) = t , employing (12) and (13), interchanging the order of the double sum, and simplifying yield i 1 2! (i - k + 1)! h(i)(t) = hαi (1 + u)α-kB - ; ;:::; (-1)i-k+1 α k i;k t2 t3 ti-k+2 k=1 X i 1α-k 1 1i = hαi 1 + - B (1!; 2!;:::; (i - k + 1)!) k t tk t i;k k=1 X (-1)i 1α i = 1 + L(i; k)hαi (1 + t)i-k ti(1 + t)i t k k=1 X (-1)i 1α i-1 = 1 + L(i; i - m)hαi (1 + t)m ti(1 + t)i t i-m m=0 X (-1)i 1α i-1 k k = 1 + L(i; i - k)hαi tj ti(1 + t)i t i-k j k=0 j=0 X X (-1)i 1α i-1 i-1 k = 1 + L(i; i - k)hαi tj ti(1 + t)i t i-k j j=0 k=j X X (-1)i 1α i-1 i-1 q = 1 + L(i; i - q)hαi tk ti(1 + t)i t i-q k k=0 q=k X X (-1)i 1α i-1 i-k i - m = 1 + L(i; m)hαi tk: ti(1 + t)i t m k k=0 m=1 X X Comparing this with the formula (5) reveals ii - 1 i-k i - m k! (α) = L(i; m)hαi : (14) k k i-k m k m=1 X From this, the identity (8) follows immediately.