ISRN Discrete Mathematics
Volume 2011 (2011), Article ID 674167, 16 pages
http://dx.doi.org/10.5402/2011/674167
Research Article

## Sequences of Numbers Meet the Generalized Gegenbauer-Humbert Polynomials

1Department of Mathematics and Computer Science, Illinois Wesleyan University, Bloomington, IL 61702, USA
2Department of Mathematical Sciences, University of Nevada Las Vegas, Las Vegas, NV 89154-4020, USA
3Department of Electrical Engineering, National Taiwan University, Taipei 106, Taiwan

Received 6 July 2011; Accepted 25 August 2011

Copyright © 2011 Tian-Xiao He et al. 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

Here we present a connection between a sequence of numbers generated by a linear recurrence relation of order 2 and sequences of the generalized Gegenbauer-Humbert polynomials. Many new and known formulas of the Fibonacci, the Lucas, the Pell, and the Jacobsthal numbers in terms of the generalized Gegenbauer-Humbert polynomial values are given. The applications of the relationship to the construction of identities of number and polynomial value sequences defined by linear recurrence relations are also discussed.

#### 1. Introduction

Many number and polynomial sequences can be defined, characterized, evaluated, and/or classified by linear recurrence relations with certain orders. A number sequence is called sequence of order 2 if it satisfies the linear recurrence relation of order 2: for some nonzero constants and and initial conditions and . In Mansour [1], the sequence defined by (1.1) is called Horadam’s sequence, which was introduced in 1965 by Horadam [2]. In [1] also the generating functions for powers of Horadam’s sequence are obtained. To construct an explicit formula of its general term, one may use a generating function, characteristic equation, or a matrix method (see Comtet [3], Hsu [4], Strang [5], Wilf [6], etc.) In [7], Benjamin and Quinn presented many elegant combinatorial meanings of the sequence defined by recurrence relation (1.1). For instance, counts the number of ways to tile an -board (i.e., board of length ) with squares (representing 1s) and dominoes (representing 2s) where each tile, except the initial one, has a color. In addition, there are colors for squares and colors for dominoes. In particular, Aharonov et al. (see [8]) have proved that the solution of any sequence of numbers that satisfies a recurrence relation of order 2 with constant coefficients and initial conditions and , called the primary solution, can be expressed in terms of the Chebyshev polynomial values. For instance, the authors show and , where and , respectively, are the Fibonacci numbers and Lucas numbers, and and are the Chebyshev polynomials of the first kind and the second kind, respectively. Some identities drawn from those relations were given by Beardon in [9]. Marr and Vineyard in [10] use the relationship to establish explicit expression of five-diagonal Toeplitz determinants. In [11], the first two authors presented a new method to construct an explicit formula of generated by (1.1). For the sake of the reader's convenience, we cite this result as follows.

Proposition 1.1 (see [11]). Let be a sequence of order 2 satisfying linear recurrence relation (1.1), and let and be two roots of of quadratic equation . Then

A sequence of the generalized Gegenbauer-Humbert polynomials is defined by the expansion (see, e.g., Comtet [3], Gould [12], Lidl et al. [13], the two authors with He et al. [14]) where , and are real numbers. As special cases of (1.3), we consider as follows (see [14]): , the Chebyshev polynomial of the second kind,, the Legendre polynomial,, the Pell polynomial,, the Fibonacci polynomial,, the Fermat polynomial of the first kind,, the Dickson polynomial of the second kind, (see, e.g., [13]),

where is a real parameter, and is the Fibonacci number. In particular, if , the corresponding polynomials are called the Gegenbauer polynomials (see [3]). More results on the Gegenbauer-Humbert-type polynomials can be found in [15] by Hsu and in [16] by the second author and Hsu, and so forth.

Similarly, for a class of the generalized Gegenbauer-Humbert polynomial sequences defined by for all with initial conditions the following theorem has been obtained in [11].

Theorem 1.2 (see [11]). Let . The generalized Gegenbauer-Humbert polynomials defined by expansion (1.3) can be expressed as

In this paper, we will use an alternative form of (1.2) to establish a relationship between the number sequences defined by recurrence relation (1.1) and the generalized Gegenbauer-Humbert polynomial sequences defined by (1.4). Our results are suitable for all such number sequences defined by (1.1) with arbitrary initial conditions and , which includes the results in [8, 9] as our special cases. Many new and known formulas of the Fibonacci, the Lucas, the Pell, and the Jacobsthal numbers in terms of the generalized Gegenbauer-Humbert polynomial values and applications of the established relationship to the construction of identities of number and polynomial value sequences will be presented in Section 3.

#### 2. Main Results

We now modify the explicit formula of the number sequences defined by linear recurrence relations of order 2. If , the first formula in (1.2) can be written as where the last step is due to and being solutions of . Noting that and , we may further write the above last expression of as

Denote and . Comparing expressions (2.2) and (1.6), we have reason to consider the following transform: for a nonzero real or complex number , we set for a certain depending on , , and , which we will find out later. Denote and ; that is, and are roots of . By adding the two equations in (2.3) side by side, we obtain . Thus, when , the equations in (2.2) hold. Meanwhile, by using , we have where . Therefore, we obtain which implies We first consider the case of .

We now substitute , , , and into (2.2) and simplify as follows: Similarly, for , we have

Therefore, we obtain our main result.

Theorem 2.1. Let sequence be defined by () with initial conditions and . Then, can be presented as (2.7) and (2.8). In particular, for , and , respectively, one has where , , , , and are the th degree Chebyshev polynomial of the second kind, the Pell polynomial, the Fibonacci polynomial, the Fermat polynomial, and the Dickson polynomial of the second kind, respectively.

For the special cases of and , we have the following corollaries.

Corollary 2.2. Let sequence be defined by () with initial conditions and . Then

Corollary 2.3. Let sequence be defined by () with initial conditions and . Then

If , then Corollary 2.2 gives the primary solutions of recurrence relation (1.1) in terms of the th degree Chebyshev polynomial of the second kind, the Pell polynomial, the Fibonacci polynomial, the Fermat polynomial, and the Dickson polynomial of the second kind, respectively. For instance, if , then are the Fibonacci numbers . Thus, where was shown in [8] and was given by Chen and Louck in [17]. From the above expressions of , we may obtain many identities. For instance, we have and so forth.

We now give another special case of Theorem 2.1 for the sequence defined by (1.1) with initial cases and .

Corollary 2.4. Let sequence be defined by () with initial conditions and . Then In addition, one has where are the Chebyshev polynomials of the first kind.

Proof. It is sufficient to prove (2.15) and (2.16). From the first formula shown in Corollary 2.4 and the recurrence relation , one easily sees From the basic relation between Chebyshev polynomials of the first and the second kinds (see, e.g., (1.7) in [18] by Mason and Handscomb), , the last expression of implies (2.15). Equation (2.16) can be proved similarly.

As an example, the Lucas number sequence defined by (1.1) with and initial conditions and has the explicit formula for its general term:

#### 3. Examples and Applications

We first give some examples of Corollary 2.2 for sequences that are primary solutions of (1.1).

Example 3.1. If and , then defined by (1.1) with initial conditions and are the Pell numbers . Thus, from Corollary 2.2, we have

Example 3.2. If and , then defined by (1.1) with initial conditions and are the Jacobsthal numbers (see Bergum et al. [19]). Thus Corollary 2.2 gives the expressions of as follows:

Example 3.3. If and , then defined by (1.1) with initial conditions and are the Mersenne numbers . From Corollary 2.2, we have

Next, we give several examples of nonprimary solutions of (1.1) by using Corollary 2.4.

Example 3.4. If and , then defined by (1.1) with initial conditions and are the Lucas numbers . Thus, besides (2.18), we have

Example 3.5. If and , then defined by (1.1) with initial conditions and are the Pell-Lucas numbers (see Example 2 in [11]). Thus, from Corollary 2.4, we obtain

Example 3.6. If and , then defined by (1.1) with initial conditions and are the Jacobsthal-Lucas numbers (see Example 2 in [11]). Thus,

Example 3.7. If and , then defined by (1.1) with initial conditions and are the Fermat numbers (see [20]). Thus, from Corollary 2.4, we obtain

Using the relationship established above, we may obtain some identities of number sequences and polynomial value sequences. Theorem 3.2 in [11] presented a generalized Gegenbauer-Humbert polynomial sequence identity: where satisfies the recurrence relation of order 2, with coefficients and , and and . Clearly (see (19) and (20) in [11]), For and , we have , where are the Fibonacci polynomials, and we can write (3.8) as where and . If , then , the Fibonacci numbers, and

Thus (3.10) yields the identity or equivalently,

Similarly, if , then , the Pell numbers, and Thus (3.10) yields the identity or equivalently,

Substituting into (3.10) and noting , where are the Jacobsthal numbers, we obtain the identity When , , the Mersenne numbers. Hence (3.10) gives .

Conversely, one may use the expressions of various number sequences in terms of the generalized Gegenbauer-Humbert polynomial sequences to construct the identities of the different generalized Gegenbauer-Humbert polynomial values such as the formulas shown in the example after Corollary 2.3.

#### Acknowledgments

P. J.-S. Shiue and T.-W. Weng would like to thank the Institute of Mathematics, Academia Sinica, Taiwan, for its financial support of the research in this paper carried out during summer 2009.

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