The Pascal Matrix Function and Its Applications to Bernoulli Numbers and Bernoulli Polynomials and Euler Numbers and Euler Polynomials
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The Pascal Matrix Function and Its Applications to Bernoulli Numbers and Bernoulli Polynomials and Euler Numbers and Euler Polynomials Tian-Xiao He ∗ Jeff H.-C. Liao y and Peter J.-S. Shiue z Dedicated to Professor L. C. Hsu on the occasion of his 95th birthday Abstract A Pascal matrix function is introduced by Call and Velleman in [3]. In this paper, we will use the function to give a unified approach in the study of Bernoulli numbers and Bernoulli poly- nomials. Many well-known and new properties of the Bernoulli numbers and polynomials can be established by using the Pascal matrix function. The approach is also applied to the study of Euler numbers and Euler polynomials. AMS Subject Classification: 05A15, 65B10, 33C45, 39A70, 41A80. Key Words and Phrases: Pascal matrix, Pascal matrix func- tion, Bernoulli number, Bernoulli polynomial, Euler number, Euler polynomial. ∗Department of Mathematics, Illinois Wesleyan University, Bloomington, Illinois 61702 yInstitute of Mathematics, Academia Sinica, Taipei, Taiwan zDepartment of Mathematical Sciences, University of Nevada, Las Vegas, Las Vegas, Nevada, 89154-4020. This work is partially supported by the Tzu (?) Tze foundation, Taipei, Taiwan, and the Institute of Mathematics, Academia Sinica, Taipei, Taiwan. The author would also thank to the Institute of Mathematics, Academia Sinica for the hospitality. 1 2 T. X. He, J. H.-C. Liao, and P. J.-S. Shiue 1 Introduction A large literature scatters widely in books and journals on Bernoulli numbers Bn, and Bernoulli polynomials Bn(x). They can be studied by means of the binomial expression connecting them, n X n B (x) = B xn−k; n ≥ 0: (1) n k k k=0 The study brings consistent attention of researchers working in combi- natorics, number theory, etc. In this paper, we will establish a matrix form of the above binomial expression using the following generalized Pascal matrix P [x] introduced by Call and Velleman in [3]: n P [x] := xn−k ; (2) k n;k≥0 which we also call the Pascal matrix function and P [1] is clearly the clas- sical Pascal matrix. Many well-known and new properties of Bernoulli numbers and Bernoulli polynomials can be obtained through this ma- T trix approach. In fact, by denoting B(x) = (B0(x);B1(x);:::) and T B = B(0) = (B1;B2;:::) , we may write (1) as B(x) = P [x]B(0): (3) P [x] is a homomorphic mapping from R to the infinite lower triangular matrices with real entries due to Theorem 2 shown [3]: Theorem 1.1 [3] (Homomorphism Theorem) For any x; y 2 R, P [x + y] = P [x]P [y]: (4) By noting ik ii − j = ; (5) k j j k − j one immediately find the (i; j) entry of the right-hand side of (4) can be written as i X i k xi−k yk−j; (6) k j k=j Pascal Matrix Function 3 while the (i; j) entry of the left-hand side of (4) is i−j i i i X i − j X ii − j (x+y)i−j = ykxi−j−k = xi−kyk−j; j j k j k − j k=0 k=j which is exactly (6) completing the proof of Theorem 1.1. In [4], iden- tity (5) is used to rederive several known properties and relationships involving the Bernoulli and Euler polynomials. In this paper, a uni- fied approach by means of P [x] is presented in the study of Bernoulli polynomials and numbers and Euler polynomials and numbers. For any square matrix A, the exponential of A is defined as the following matrix in a series form: 1 1 X 1 eA = I + A + A2 + A3 + ··· = Ak: 2! 3! k! k=0 We say the series converges for every A if each entry of eA converges. From [13] and the definition of eA, we have (α+β)A αA βA e = e e for any α; β 2 R; (eA)−1 = e−A; tA tA tA Dte = Ae = e A: (7) i i−j For n 2 N [ 0, denote Pn[x] = ( j x )0≤i;j≤n. [3] shows there exists xHn a unique Hn = (hi;j)0≤i;j≤n such that Pn[x] = e . In fact, from (7), one may have H = D exHn = D P [x] : (8) n x x=0 x n x=0 Denote H = (hi;j)0≤i;j. Then (4) of Theorem 1.1 can be proved in one line: P [x + y] = e(x+y)H = exH eyH = P [x]P [y]: It is easy to see that the entry hi;j of H and Hn, n = 0; 1;:::, are i if i = j + 1; h = (H) = ; (9) i;j i;j 0 otherwise: 4 T. X. He, J. H.-C. Liao, and P. J.-S. Shiue and the entry of Hk are i! if i = j + k; (Hk) = j! (10) i;j 0 otherwise: k Hence, Hn = 0 if k ≥ n. In next section, we will define new matrix functions from Pn[x], which and the matrix relationship (3) between the Bernoulli numbers and the Bernoulli polynomials through P [x] will be used to re-build some well-known properties of the Bernoulli numbers and the Bernoulli polynomials in Section 3. Finally, in Section 4, a similar approach is used to study the Euler numbers and Euler polynomials and their relationships with Bernoulli numbers and Bernoulli polynomials. 2 Matrix functions Ln[x] and L¯n[x] related to Pn[x] We now present the integrals of P [x]. From [1] (on Page 240), there R 1 holds expression of the linear mapping Ln : Pn[x] 7! 0 Pn[x]dx from matrix functions to matrices as n−1 Z 1 X Hk L = P [x]dx = n ; (11) n n (k + 1)! 0 k=0 xHn which is simply from the fact Pn[x] = e and the last formula in (7). Notice that L is a nonsingular lower triangular matrix with all main diagonal entries equal to 1. We now extend matrices (11) to matrix functions n−1 Z x Z x X Hk L [x] := P [t]dt = etHn dt = n xk+1; (12) n n (k + 1)! 0 0 k=0 R x which can be considered a mapping Ln[x]: Pn[t] 7! 0 Pn[t]dt associ- ated with any x 2 R. It is obvious Ln[1] ≡ Ln defined in (11). From (10) and noting k = i − j, the entries of Ln[x] can be given as 1 ixi−j+1; if i ≥ j ≥ 0; (L [x]) = i−j+1 j (13) n ij 0; otherwise: Pascal Matrix Function 5 Particularly, 1 i; if i ≥ j; (L ) = i−j+1 j (14) n ij 0; otherwise: We now give some properties of Ln[x] and its relationships with Pn[x]. Proposition 2.1 Let matrices Hn, Pn[x], and Ln[x] be defined as above. Then there holds HnLn[x] = Ln[x]Hn = Pn[x] − In: (15) Proof. By transferring the indexes, we have n X Hk H L [x] = L [x]H = n xk n n n n k! k=1 n X Hk = n xk − I = P [x] − I : k! n n n k=0 Inspired by [1], we define ¯ −1 Ln = Dn(−1)LnDn(−1) ; and ¯ −1 Ln[x] = Dn(−1)Ln[x]Dn(−1) ; (16) n−1 ¯ ¯ where Dn(−1) = diag(1; −1; 1; −1;:::; (−1) ). Hence, Ln = Ln[1]. −1 Note. Since Dn(−1)HnDn(−1) = −Hn, we have n−1 k −1 X Dn(−1)H Dn(−1) D (−1)P [x]D (−1) = n xk n n n k! k=0 n−1 k X (Dn(−1)HnDn(−1)) = xk k! k=0 n−1 k n−1 k X (−Hn) X (Hn) = xk = (−x)k = P [−x]: k! k! n k=0 k=0 ¯ The relationship between Ln[x] and Ln[x] can be shown below. 6 T. X. He, J. H.-C. Liao, and P. J.-S. Shiue ¯ Proposition 2.2 Let Ln[x] and Ln[x] be defined as before. Then there hold ¯ ¯ Ln[x] = −Ln[−x];Ln[x] = −Ln[−x]; and (17) ¯ ¯ Ln = −Ln[−1];Ln = −Ln[−1]: (18) Proof. From (16), Z x Z x Z −x ¯ −1 Ln[x] = Dn(−1) Pn[t]dt Dn(−1) = Pn[−t]dt = − Pn[t]dt; 0 0 0 which implies (17), and (18) follows as well. ¯ Using the above relationship between Ln[x] and Ln[x] and Proposi- tion 2.1, we immediately have ¯ Proposition 2.3 Let Hn, Pn[x], and Ln[x] be defined as before. Then n−1 k X (−Hn) L¯ [x] = xk+1; and (19) n (k + 1)! k=0 ¯ ¯ −HnLn[x] = −Ln[x]Hn = P [−x] − In; (20) n−1 k n−1 k X (−Hn) X H (−x)k+1 = − n xk+1: (21) (k + 1)! (k + 1)! k=0 k=0 Proof. From (16) and (17) ¯ −1 Ln[x] = Dn(−1)Ln[x]Dn(−1) n−1 k X (Dn(−1)HnDn(−1)) = xk+1 (k + 1)! k=0 n−1 k X (−Hn) = xk+1; (k + 1)! k=0 i.e., formula (19). Again, (17) and (15) yield Pascal Matrix Function 7 ¯ −HnLn[x] = HnLn[−x] = Pn[−x] − In and ¯ −Ln[x]Hn = Ln[−x]Hn = Pn[−x] − In: Substituting x = −x into (12) and noting (17) and (19), we have n−1 k n−1 k X (−Hn) X H xk+1 = L¯ [x] = −L [−x] = − n (−x)k+1; (k + 1)! n n (k + 1)! k=0 k=0 which implies (21).