Hacettepe Journal of Mathematics and Statistics Volume 46 (3) (2017), 361 372 Incomplete k-Pell, k-Pell-Lucas and modied k-Pell numbers Paula Catarino∗† and Helena Camposz Abstract In this paper, it is dened the incomplete k-Pell, k-Pell-Lucas and Mod- ied k-Pell numbers, it is studied the recurrence relations, some prop- erties of these sequences of integers and their generating functions. Keywords: Incomplete Pell numbers, Incomplete Pell-Lucas numbers, Incom- plete Modied Pell numbers, k-Pell, k-Pell-Lucas and Modied k-Pell numbers. 2000 AMS Classication: 11B39, 11B83, 65Q30, 05A15. Received : 20.10.2015 Accepted : 11.04.2016 Doi : 10.15672/HJMS.20164518616 1. Introduction The Fibonacci and Lucas sequences are the sequences of positive integers that have been studied over several years. Many authors have investigated about these sequence [18], [21], [22], [23], [32], [35] and also some generalizations of them have been a great topic of research [1], [2], [3], [8], [12], [13], [14], [16]. These sequences are examples of a sequences dened by a recurrence relation of second order. It is well known that the Fibonacci sequence fFngn is dened by the following recurrence relation Fn = Fn−1 + Fn−2; n ≥ 2 with F0 = 0 and F1 = 1. In the case of the Lucas sequence fLngn we have Ln = Ln−1 + Ln−2; n ≥ 2 with L0 = 2 and L1 = 1. ∗Department of Mathematics, University of Trás-os-Montes e Alto Douro, UTAD 5000-801, Vila Real, Portugal, Member of the Portuguese research centres CMAT-Polo of UTAD and Lab-DCT of CIDTFF. Email: [email protected] yCorresponding Author. zDepartment of Mathematics, University of Trás-os-Montes e Alto Douro, UTAD 5000-801, Vila Real, Portugal, Member of the Portuguese research centre Lab-DCT of CIDTFF. Email: [email protected] 362 As we have mentioned above, many generalizations of Fibonacci (and Lucas) sequence has been investigated. One of these generalizations is the k−Fibonacci sequence fFk;ngn with k any integer. This sequence is dened by Fk;n = kFk;n−1 + Fk;n−2; n ≥ 2 with Fk;0 = 0 and Fk;1 = 1. The sequence of Pell numbers is a particular case of this sequence, considering k = 2. Hence the Pell sequence, denoted by fPngn, is dened by the recursive sequence given by (1.1) Pn = 2Pn−1 + Pn−2; n ≥ 2 with initial terms P0 = 0 and P1 = 1. If we modify the values of the initial conditions of this sequence, we obtain two more sequences called by Pell-Lucas and the Modied Pell sequences. The sequence of Pell-Lucas numbers is dened by the recursive sequence given by (1.2) Qn = 2Qn−1 + Qn−2; n ≥ 2 with initial terms Q0 = Q1 = 2, and the Modied Pell sequence fqngn is given by the following recursive relation (1.3) qn = 2qn−1 + qn−2; n ≥ 2 with initial terms q0 = q1 = 1. Some identities related with these sequences involving sums formulae for products involving its terms have been studied by several authors (see, for example, [17], among others). Catarino in [5] studied a generalization of Pell sequence, as well as in [6] and in [7], where Catarino and Vasco studied generalizations of Pell-Lucas and Modied Pell sequences, respectively. Also in [30], the authors studied some identities and norms of Hankel matrices whose entries are elements of these sequences, as well as in [31], we can nd some properties involving sums of products with its terms. Such generalizations are dened by a recurrence relations of second order given, for any positive real number k, by: (1.4) Pk;n = 2Pk;n−1 + kPk;n−2; n ≥ 2 with initial terms Pk;0 = 0 and Pk;1 = 1, for k-Pell sequence; (1.5) Qk;n = 2Qk;n−1 + kQk;n−2; n ≥ 2 with initial terms Qk;0 = Qk;1 = 2, for k-Pell-Lucas sequence; and (1.6) qk;n = 2qk;n−1 + kqk;n−2; n ≥ 2 with initial terms qk;0 = qk;1 = 1, for Modied k-Pell sequence. Note that in the particular case where k = 1, (1.4)-(1.5)-(1.6) reduces to (1.1)-(1.2)-(1.3), respectively. Also, Djordjevi¢ in [9] considers the convolution of the generalized Pell and Pell-Lucas (s) (s) numbers Pn;m and Qn;m respectively, for a nonnegative integers n; s and a natural number (0) m. For s = 0, the sequence Pn;m represents the generalized Pell numbers Pn;m, and the (0) sequence Qn;m represents the generalized Pell-Lucas numbers Qn;m. In addition, for s = 0 and the numbers (0) and (0) are Pell and Pell-Lucas numbers, respectively. m = 2 Pn;2 Qn;2 (s) In this paper the author presents the recurrence relation of the sequences fPn;mg and (s) fQn;mg and some properties of these sequences are stated. There are many directions in order to develop our investigation through to these type of special numbers. For example, the mathematical term incomplete on Fibonacci, Lu- cas and Tribonacci numbers and polynomials has been considered. For studies about 363 the incomplete Fibonacci and Lucas numbers and their generating functions and prop- erties, see, for example, [15] and [24], and for the incomplete Tribonacci numbers, see, for example, [26] and [33]. The incomplete generalized Fibonacci and Lucas numbers are presented in [10] and the incomplete generalized Jacobsthal and Jacobsthal-Lucas numbers in [11]. We may also refer to [25], [28], [29] and [34], among others. Motivated essentially by these works, we aim to introduce and study here the analo- gously incomplete version of each one of these three classes of numbers: k-Pell, k-Pell- Lucas and Modied k-Pell numbers. We begin recalling some properties involving these sequences of numbers and then, for each one, we introduce and present some properties and the generating function. 2. A summary of some properties of k-Pell, k-Pell-Lucas and Mod- ied k-Pell numbers The characteristic equation associated with the recurrence relations (1.4), (1.5) and p p (1.6) is 2 whose roots are and . Clearly, r − 2r − k = 0 p r1 = 1 + 1 + k r2 = 1 − 1 + k r1 + r2 = 2; r1 − r2 = 2 1 + k and r1r2 = −k. The k-Pell, k-Pell-Lucas and Modied k-Pell numbers verify the following properties (see, [5], [6] and [7] for the proofs), respectively: n n n n r1 −r2 n n r1 +r2 • Binet's formula: Pk;n = ; Qk;n = r + r ; qk;n = . r1−r2 1 2 2 Generating function: −t ; 2−2t ; 1−t . • f(t) = 1−2t−kt2 g(t) = 1−2t−kt2 h(t) = 1−2t−kt2 The Pell, Pell-Lucas and Modied Pell numbers verify the corresponding following properties (see, [19] and [20] for the proofs), respectively: rn−rn rn+rn • Binet's formula: P = 1 p 2 ; Q = rn + rn; q = 1 2 . n 2 2 n 1 2 n 2 Generating function: −t ; 2−2t ; 1−t . • f(t) = 1−2t−t2 g(t) = 1−2t−t2 h(t) = 1−2t−t2 and n−1 b c n − 1 − j • Explicit Formula: P = P 2 2n−1−2j (see, for example, Theorem n j=0 j b n c n − j 9.1 in [20], p.173, or, see also (1.1) for s = 0 and m = 2 in [9]); Q = P 2 n 2n−2j ; n j=0 n−j j and since 2qn = Qn; n ≥ 0, then the explicit formula to Modied Pell number is given b n c n − j by q = 1 Q = P 2 n 2n−2j−1. n 2 n j=0 n−j j 3. The incomplete k-Pell numbers and some properties 3.1. Denition. With k any integer, the incomplete k-Pell numbers are dened by l l X n − 1 − j n−1−2j j n − 1 (3.1) P := 2 k ; (0 ≤ l ≤ b c; n 2 ): k;n j 2 N j=0 Note that n−1 b 2 c Pk;n = Pk;n and some special cases of (3.1) are: 0 n−1 Pk;n = 2 ; (n ≥ 1); 1 n−1 n−3 Pk;n = 2 + k(n − 2)2 ; (n ≥ 3); (n − 4)(n − 3) P 2 = 2n−1 + k(n − 2)2n−3 + 2n−5k2; (n ≥ 5); k;n 2 364 ( n−2 n−3 2 even b 2 c Pk;n − nk ; (n ) Pk;n = n−1 (n ≥ 3): Pk;n − k 2 ; (n odd) By Denition 3.1, in the Table 1 we present a few incomplete k-Pell numbers. nnj j = 0 j = 1 j = 2 j = 3 n = 1 1 n = 2 2 n = 3 4 4 + k n = 4 8 8 + 4k n = 5 16 16 + 12k k2 + 12k + 16 n = 6 32 32 + 32k 6k2 + 32k + 32 n = 7 64 64 + 80k 24k2 + 80k + 64 k3 + 24k2 + 80k + 64 n = 8 128 128 + 192k 80k2 + 192k + 128 8k3 + 80k2 + 192k + 128 Table 1. The incomplete l for Pk;n 1 ≤ n ≤ 8 In the next result we present the recurrence relation veried by these numbers followed by other results which reveals some properties of the numbers l . Pk;n 3.2. Proposition. With k any integer, the recurrence relation of the incomplete k-Pell numbers l is Pk;n n − 2 (3.2) P l+1 = 2P l+1 + kP l ; (0 ≤ l ≤ ; n 2 ): k;n+2 k;n+1 k;n 2 N Proof. By Denition 3.1 we get l+1 l l+1 l X n − j n−2j j X n − 1 − j n−1−2j j 2P + kP = 2 2 k + k 2 k k;n+1 k;n j j j=0 j=0 l+1 l X n − j n−2j+1 j X n − 1 − j n−1−2j j+1 = 2 k + 2 k j j j=0 j=0 l+1 l+1 X n − j n−2j+1 j X n − j n−2j+1 j = 2 k + 2 k j j − 1 j=0 j=1 l+1 X n − j n − j n+1−2j j = + 2 k j j − 1 j=0 l+1 X n − j + 1 n−2j+1 j = 2 k j j=0 l+1 = Pk;n+2; as required.