Volume 15, Number 1, Pages 163–174 ISSN 1715-0868
CLOSED FORMULAS AND IDENTITIES FOR THE BELL POLYNOMIALS AND FALLING FACTORIALS
FENG QI, DA-WEI NIU, DONGKYU LIM, AND BAI-NI GUO
Abstract. The authors establish a pair of closed-form expressions for special values of the Bell polynomials of the second kind for the falling factorials, derive two pairs of identities involving the falling factorials, find an equivalent expression between two special values for the Bell polynomials of the second kind, and present five closed-form expressions for the (modified) spherical Bessel functions.
1. Motivations The Bell polynomials of the second kind, also known as partial Bell poly- nomials, denoted by Bn,k(x1, x2, . . . , xn−k+1) for n ≥ k ≥ 0, are defined in [1, p. 134, Theorem A] by
n−k+1 X n! Y xi `i (1.1) B (x , x , . . . , x ) = . n,k 1 2 n−k+1 Qn−k+1 i! 1≤i≤n−k+1 i=1 `i! i=1 `i∈{0}∪N Pn−k+1 i=1 i`i=n Pn−k+1 i=1 `i=k The Fa`adi Bruno formula [1, p. 139, Theorem C] can be described in terms of the Bell polynomials of the second kind Bn,k(x1, x2, . . . , xn−k+1) by
n dn X (1.2) f ◦ h(x) = f (k)(h(x)) B h0(x), h00(x), . . . , h(n−k+1)(x). dxn n,k k=0
Received by the editors April 17, 2019, and in revised form August 28, 2019. 2010 Mathematics Subject Classification. Primary 11B83; Secondary 05A10, 11B65, 12E10, 26C05, 33C10, 43A90. Key words and phrases. closed-form expression; Bell polynomial of the second kind; falling factorial; special value; identity; equivalent expression; spherical Bessel function; modified spherical Bessel function; Fa`adi Bruno formula. The third author was supported by the National Research Foundation of Korea (Grant No. 2018R1D1A1B07041846) Corresponding author: Bai-Ni Guo.
c 2020 University of Calgary 163 164 F. QI, D.-W. NIU, D. LIM, AND B.-N. GUO
By (1.1), we can easily deduce that, for n ≥ k ≥ 0, d d2 dn−k+1 B (1, 0,..., 0) = B x, x, . . . , x n,k n,k dx dx2 dxn−k+1 (1.3) ( 0 1, n = k; = = n − k 0, n 6= k. In [10, Theorem 5.1] and [16, Section 3], it was established that the Bell polynomials of the second kind Bn,k(x1, x2, . . . , xn−k+1) satisfy 1 n! k (1.4) B (x, 1, 0,..., 0) = x2k−n, n,k 2n−k k! n − k 0 p where n ≥ k ≥ 0, 0 = 1, and q = 0 for q > p ≥ 0. Since (1.5) 2 n−k+1 k n Bn,k abx1, ab x2, . . . , ab xn−k+1 = a b Bn,k(x1, x2, . . . , xn−k+1) for n ≥ k ≥ 0, see [1, p. 135], we can rearrange (1.4) as d d2 dn−k+1 B x2, x2,..., x2 = B (2x, 2, 0,..., 0) n,k dx dx2 dxn−k+1 n,k n! k = 2k B (x, 1, 0,..., 0) = (2x)2k−n n,k k! n − k for n ≥ k ≥ 0. This means that one can combine the Fa`adi Bruno for- mula (1.2) with the formula (1.4) to compute the nth derivative for functions of the type f ax2 + bx + c, such as
2 e±x , sin x2, cos x2, ln 1 ± x2, (1.6) 1 ± x2α, arcsin x, arccos x, arctan x, and to investigate the generating functions 1 1 1 √ , , √ , 1 − 2xt + t2 1 − 2xt + t2 1 − 6x + x2 √ √ 1 − x − x2 − 6x + 1 1 + x − x2 − 6x + 1 , , 2√x 4 2 z 1 − x − 1 − 2x − 3x 2 2e , e2xt−t , 2x2 e2z + 1 of the Legendre polynomials, the Chebyshev polynomials of the second kind, central Delannoy numbers, the large and little Schr¨oder numbers, the Motzkin numbers, the Hermite polynomials, the Euler numbers, and the Rodrigues formulas for the Chebyshev polynomials of the first and second kinds. This idea has been carried out and applied in [7, 8, 10, 14, 16] and closely related references therein. Now it is natural and significant to ask the following question: how to α compute the nth derivative of functions of the type f(x ) for α ∈ R and to FORMULAS AND IDENTITIES FOR BELL POLYNOMIALS 165 apply the results? To answer such a question, by virtue of the Fa`adi Bruno formula (1.2), we need to calculate d d2 dn−k+1 B xα, xα,..., xα n,k dx dx2 dxn−k+1 α−1 α−2 α−(n−k+1) = Bn,k hαi1x , hαi2x ,..., hαin−k+1x kα−n = x Bn,k(hαi1, hαi2,..., hαin−k+1), where we used identity (1.5) and n−1 ( Y α(α − 1) ··· (α − n + 1), n ≥ 1; (1.7) hαin = (α − k) = 1, n = 0 k=0 is called the falling factorial. In recent years, the first author and his coauthors discovered and applied many closed-form expressions of special values for the Bell polynomials of the second kind Bn,k(x1, x2, . . . , xn−k+1) in the papers [6, 7, 8, 9, 15] and closely related references therein. In this paper, we will establish a pair of closed-form expressions for the Bell polynomials of the second kind Bn,k(hαi1, hαi2,..., hαin−k+1), derive two pairs of identities involving the falling factorials hαi`, find an equivalent expression of Bn,k(hαi1, hαi2,..., hαin−k+1), and present five closed-form ex- (1) pressions for the (modified) spherical Bessel functions jn(z), yn(z), in (z), (2) in (z), and kn(z).
2. A pair of closed-form expressions for Bell polynomials Now we state a pair of closed-form expressions for the Bell polynomials of the second kind Bn,k(hαi1, hαi2,..., hαin−k+1) and its proof. Theorem 2.1. For n ≥ k ≥ 0 and α ∈ R, the Bell polynomials of the second kind Bn,k(hαi1, hαi2,..., hαin−k+1) can be computed by k (−1)k X k (2.1) B (hαi , hαi ,..., hαi ) = (−1)` hα`i n,k 1 2 n−k+1 k! ` n `=0 and k X Bn,`(hαi1, hαi2,..., hαin−`+1) hαkin (2.2) = . (k − `)! k! `=0 Proof. In [1, p. 133], it is listed that
∞ !k ∞ 1 X tm X tn (2.3) x = B (x , x , . . . , x ) k! m m! n,k 1 2 n−k+1 n! m=1 n=k α−m for k ≥ 0. Taking xm = hαimx for α ∈ R in (2.3) leads to 166 F. QI, D.-W. NIU, D. LIM, AND B.-N. GUO
∞ X tn B hαi xα−1, hαi xα−2,..., hαi xα−(n−k+1) n,k 1 2 n−k+1 n! n=k k αk " ∞ m# αk α k x X hαim t x t = = 1 + − 1 . k! m! x k! x m=1 α 1 Taking a = x , b = x , and xi = hαii for 1 ≤ i ≤ n−k+1 in the identity (1.5), we obtain α−1 α−2 α−(n−k+1) Bn,k hαi1x , hαi2x ,..., hαin−k+1x kα−n = x Bn,k(hαi1, hαi2,..., hαin−k+1). Consequently, it follows that ∞ α k X tn xαk t xkα−n B (hαi , hαi ,..., hαi ) = 1 + − 1 n,k 1 2 n−k+1 n! k! x n=k which can be rearranged as ∞ X sn [(1 + s)α − 1]k B (hαi , hαi ,..., hαi ) = n,k 1 2 n−k+1 n! k! n=k k k 1 X k (−1)k X k = (−1)k−` (1 + s)α` = (−1)` (1 + s)α`, k! ` k! ` `=0 `=0 t where s = x . Differentiating p ≥ k ≥ 0 times with respect to s at both ends of the above equality gives ∞ X sn−p B (hαi , hαi ,..., hαi ) n,k 1 2 n−k+1 (n − p)! n=k k (−1)k X k = (−1)` hα`i (1 + s)α`−p. k! ` p `=0 Further letting s → 0 on both sides of the above equality results in k (−1)k X k B (hαi , hαi ,..., hαi ) = (−1)` hα`i . p,k 1 2 n−k+1 k! ` p `=0 The formula (2.1) thus follows. The binomial inversion theorem [1, pp. 143–144] reads n n X n X n (2.4) s = S if and only if S = (−1)n−k s n k k n k k k=0 k=0 for n ≥ 0, where {sn, n ≥ 0} and {Sn, n ≥ 0} are sequences of complex numbers. Applying this theorem to (2.1) readily produces (2.2). The proof of Theorem 2.1 is complete. FORMULAS AND IDENTITIES FOR BELL POLYNOMIALS 167
Let
n−1 ( Y x(x + 1) ··· (x + n − 1), n ≥ 1; (x)n = (x + `) = 1, n = 0 `=0 denote the rising factorial of x ∈ R. Since
n n (−x)n = (−1) hxin and h−xin = (−1) (x)n, we can now rewrite Theorem 2.1 in terms of the rising factorial (x)n as follows.
Corollary 2.1. For n ≥ k ≥ 0 and α ∈ R, the Bell polynomials of the second kind Bn,k((α)1, (α)2,..., (α)n−k+1) can be computed by k (−1)k X k B ((α) , (α) ,..., (α) ) = (−1)` (α`) n,k 1 2 n−k+1 k! ` n `=0 and k X Bn,`((α)1, (α)2,..., (α)n−`+1) (αk)n = . (k − `)! k! `=0 3. Two pairs of identities involving falling factorials We now recover formula (1.3) from Theorem 2.1 as follows. Replacing α by 1 in (2.1) leads to k (−1)k X k B (h1i , h1i ,..., h1i ) = (−1)` h`i n,k 1 2 n−k+1 k! ` n `=0 which is equivalent to k n (−1)k X k X B (1, 0,..., 0) = (−1)` s(n, m)`m n,k k! ` `=0 m=0 n " k # X (−1)k X k = s(n, m) (−1)` `m k! ` m=0 `=0 n X 0 = s(n, m)S(m, k) = , n − k m=0 where s(n, k) and S(n, k) denote the Stirling numbers of the first and second kinds and we used the formulas n k X 1 X k hxi = s(n, k)xk,S(n, k) = (−1)k−` `n, n k! ` k=0 `=1 and n X 0 s(n, α)S(α, k) = n − k α=0 168 F. QI, D.-W. NIU, D. LIM, AND B.-N. GUO in [1, p. 206] and [17, p. 171, Eq. (12.19)]. Formula (1.3) is thus recovered. Now we state and prove two pairs of identities involving falling factorials. Theorem 3.1. For n ≥ k ≥ 0, we have k X k n! k (3.1) (−1)k−` h2`i = ` n 2n−2k n − k `=0 and k X k ` 2n (3.2) 4` = h2ki . ` n − ` n! n `=0 Proof. Replacing α by 2 in (2.1) and making use of (1.7) and (1.5) in se- quence, we derive
Bn,k(h2i1, h2i2,..., h2in−k+1) = Bn,k(2, 2, 0,..., 0) k (−1)k X k = 2k B (1, 1, 0,..., 0) = (−1)` h2`i . n,k k! ` n `=0 Comparing this with (1.4) for x = 1 yields (3.1). Applying the binomial inversion theorem recited in (2.4) to (3.1) leads to (3.2). The proof of Theorem 3.1 is complete. Theorem 3.2. For n ≥ k ≥ 0, we have k X k ` k![2(n − k) − 1]!!2n − k − 1 (3.3) (−1)` = (−1)n ` 2 2n 2(n − k) `=0 n and k X [2(n − `) − 1]!!2n − ` − 1 2n k (3.4) (−1)` = (−1)n , (k − `)! 2(n − `) k! 2 `=0 n where the double factorial of negative odd integers −(2n + 1) is defined by (−1)n 2nn! (−2n − 1)!! = = (−1)n , n ≥ 0. (2n − 1)!! (2n)! Proof. The first paragraph in [14, Theorem 4] reads that the Bell polyno- mials of the second kind Bn,k(x1, x2, . . . , xn−k+1) satisfy