Notes on Number Theory and Discrete Mathematics Print ISSN 1310–5132, Online ISSN 2367–8275 Vol. 24, 2018, No. 3, 47–55 DOI: 10.7546/nntdm.2018.24.3.47-55

2-Fibonacci in the family of Fibonacci numbers

Engin Özkan1, Merve Ta¸stan1 and Ali Aydogdu˘ 2

1 Department of Mathematics, University of Erzincan Binali Yıldırım Faculty of Arts and Sciences, Yalnızbag˘ Campus, 24100, Erzincan, Turkey e-mails: [email protected], [email protected] 2 Department of Mathematics, University of Beykent Ayazaga˘ Campus, Ayazaga-˘ Maslak, Sarıyer, 34485, Istanbul,˙ Turkey e-mail: [email protected]

Received: 3 April 2018 Revised: 17 August 2018 Accepted: 19 August 2018

Abstract: In the present study, we define new 2-Fibonacci polynomials by using terms of a new family of Fibonacci numbers given in [4]. We show that there is a relationship between the coefficient of the 2-Fibonacci polynomials and Pascal’s triangle. We give some identities of the 2-Fibonacci polynomials. Afterwards, we compare the polynomials with known Fibonacci poly- nomials. We also express 2-Fibonacci polynomials by the Fibonacci polynomials. Furthermore, we prove some theorems related to the polynomials. Also, we introduce the derivative of the 2-Fibonacci polynomials. Keywords: Fibonacci numbers, Fibonacci polynomials, Generalized Fibonacci polynomials. 2010 Mathematics Subject Classification: 11B39.

1 Introduction

Fibonacci numbers are of a great importance in mathematics and they have many important ap- plications to diverse fields (see, e.g., [7]). The Fibonacci numbers 퐹푛 are given by the

퐹푛+1 = 퐹푛 + 퐹푛−1 , 푛 ≥ 1

47 with the initial conditions 퐹0 = 0 and 퐹1 = 1. It is well known that the Fibonacci numbers are given by Binet’s formula

1 (︀ 푛+1 푛+1)︀ 퐹푛 = √ 훼 − 훽 , 푛 = 0, 1, 2,..., 5 √ √ where 훼 = (︀1 + 5)︀ /2 and 훽 = (︀1 − 5)︀ /2. The Fibonacci polynomials were studied in 1883 by E. C. Catalan and E. Jacobsthal. The

Fibonacci polynomials 퐹푛(푥) studied by Catalan are defined by the recurrence relation

퐹푛+2 (푥) = 푥퐹푛+1 (푥) + 퐹푛 (푥) , 푛 ≥ 1

with initial condition 퐹1(푥) = 1, 퐹2(푥) = 푥. The polynomials 퐽푛(푥) studied by Jacobsthal are defined by

퐽푛+2(푥) = 퐽푛+1(푥) + 푥퐽푛(푥),

where 퐽0(푥) = 퐽1(푥) = 1 and 푛 ≥ 0. It is well known that the Fibonacci polynomials bear relation with the Fibonacci numbers,

where 퐹푛(1) = 퐹푛 is the 푛-th . Some identities for the Fibonacci polynomials are given by Swamy [13]. Hoggatt and Lind [3] made a similar symbolic substitution of certain sequence into the Fibonacci polynomials. Hoggatt et al. [4] obtained some more identities for Fibonacci and Lucas Polynomials. Ivie [6] introduced a general type of Q-matrix for the gener- alized Fibonacci sequence. Hoggatt and Bicknell [5] generalized the Fibonacci polynomials and their relationship to diagonals of Pascal’s triangle. Falcon and Plaza [2] presented the derivatives of 푘-Fibonacci polynomials as convolution of the 푘-Fibonacci polynomials and proved many re- lations for the derivatives of Fibonacci polynomials. Nalli and Haukkanen [10] introduced the ℎ(푥)-Fibonacci polynomials, where ℎ(푥) is a with real coefficient. C. Berg [1] gave Fibonacci numbers and orthogonal polynomials. Tuglu˘ et al. [14] introduced generalized bi- variate Fibonacci and Lucas 푝-polynomails. Lee and Asci [8] defined a new generalization of Fibonacci polynomials called (푝, 푞)-Fibonacci polynomials and obtained combinatorial identi- ties. Panwar et al. [11] described sums of generalized Fibonacci polynomials. Ramirez [12] introduced the convolved ℎ(푥)-Fibonacci polynomials. Ye and Zhang [15] gave a common gen- eralization of convolved generalized Fibonacci and Lucas polynomials. The paper is organized as follows. In Section 2, we briefly review some basic concepts about the 푘-Fibonacci numbers and Fibonacci polynomials. In Section 3, we define new 2-Fibonacci polynomials by using terms of a new family of Fibonacci numbers given in [9] and show that the relationship between Pascal’s Triangle and the coefficients of the 2-Fibonacci polynomials. Also, we present 2-Fibonacci polynomials using the Fibonacci polynomials and give the derivatives of the 2-Fibonacci polynomials.

2 Materials and methods

It is well-known that the Fibonacci polynomials: 퐹푛(푥) for 푛 = 0, 1, 2,... studied by Catalan are defined by the recurrence relation

퐹푛+2(푥) = 푥퐹푛+1(푥) + 퐹푛(푥),

48 where 퐹0(푥) = 0, 퐹1(푥) = 1, 퐹2(푥) = 푥 and 푛 ≥ 1, the Fibonacci polynomials are generated by matrix (︃ )︃ (︃ )︃ 푥 1 푛 퐹푛+1 (푥) 퐹푛 (푥) 푄2 = , 푄2 = . 1 0 퐹푛 (푥) 퐹푛−1 (푥)

푡ℎ This can be verified quite easily by mathematical induction. If 푥 = 1, then 퐹푛(1) is the 푛 푡ℎ Fibonacci number 퐹푛, and if 푥 = 2, then 퐹푛(2) is the 푛 푃푛. If one writes Pascal’s triangle in left-justified form, one obtains the Fibonacci numbers by adding the elements along the rising diagonals. Indeed, those elements are the coefficients of the Fibonacci polynomials. That is

[(푛−1)/2] ∑︁ (︂푛 − 푗 − 1)︂ 퐹 (푥) = 푥푛−2푗−1, 푛 푗 푗=0

(︀푛)︀ where [푥] is the greatest integer contained in 푥, and 푗 is a binomial coefficient. The first few Fibonacci polynomials and their coefficient array are shown as follows:

Fibonacci polynomials Coefficients array

퐹1 (푥) = 1 1

퐹2 (푥) = 푥 1 2 퐹3 (푥) = 푥 + 1 1 1 3 퐹4 (푥) = 푥 + 2푥 1 2 4 2 퐹5 (푥) = 푥 + 3푥 + 1 1 3 1 5 3 퐹6 (푥) = 푥 + 4푥 + 3푥 1 4 3 6 4 2 퐹7 (푥) = 푥 + 5푥 + 6푥 + 1 1 5 6 1 7 5 3 퐹8 (푥) = 푥 + 6푥 + 10푥 + 4푥 1 6 10 4 . . . . Table 1. The first few Fibonacci polynomials and the array of their coefficients

Considering the rule of formation of Fibonacci polynomials, we write the polynomials in

descending order. We get the coefficient of the 푘-th term of 퐹푛(푥) by adding the coefficient of the

푘-th term of 퐹푛−1(푥) and the (푘 − 1)-st term of 퐹푛−2(푥). Hence, the array of coefficients formed has the same rule of formation as Pascal’s triangle when it is written in left-justified form, except that each column is moved one line lower. Therefore, the coefficients formed are those elements that appear along the diagonals formed by beginning in the left-most column and preceding up one and right one throughout the left-justified Pascal triangle [12].

Definition 2.1. [9] Let 푛 and 푘 ̸= 0 be natural numbers, then there exist unique numbers 푚 and (푘) 푟 such that 푛 = 푚푘 + 푟 (0 ≤ 푟 < 푘). The generalized 푘-Fibonacci numbers 퐹푛 are defined by

(푘) 1 (︀ 푚+2 푚+2)︀푟 (︀ 푚+1 푚+1)︀푘−푟 퐹푛 = √ 푘 훼 − 훽 훼 − 훽 , 푛 = 푚푘 + 푟, (︀ 5)︀

49 √ √ where 훼 = (︀1 + 5)︀ /2 and 훽 = (︀1 − 5)︀ /2.

The first numbers of the family for 푘 = 2 is as follows:

(2) {퐹푛 } = {1, 1, 1, 2, 4, 6, 9, 15, 25, 40, 64, ...}.

It is well known that the relation of generalized 푘-Fibonacci and Fibonacci numbers is

(푘) 푘−푟 푟 퐹푛 = (퐹푚) (퐹푚+1) , where 푛 = 푚푘 + 푟. When 푘 = 1 in the last equation, we get that 푚 = 푛 and 푟 = 0. Therefore, (1) 퐹푛 = 퐹푛.

3 Main results

Definition 3.1. The 2-Fibonacci polynomials are defined by

(2) (2) (2) 퐹푛+1 (푥) = 푥퐹푛 (푥) + 퐹푛−1 (푥) , 푛 ≥ 1,

(2) (2) where 퐹0 (푥) = 1 and 퐹1 (푥) = 1. The first few 2-Fibonacci polynomials are displayed below as well as the array of their coef- ficients. 2-Fibonacci polynomials Coefficients array (2) 퐹1 (푥) = 1 1 (2) 퐹2 (푥) = 1 + 푥 1 1 (2) 2 퐹3 (푥) = 1 + 푥 + 푥 1 1 1 (2) 2 3 퐹4 (푥) = 1 + 2푥 + 푥 + 푥 1 2 1 1 (2) 2 3 4 퐹5 (푥) = 1 + 2푥 + 3푥 + 푥 + 푥 1 2 3 1 1 (2) 2 3 4 5 퐹6 (푥) = 1 + 3푥 + 3푥 + 4푥 + 푥 + 푥 1 3 3 4 1 1 (2) 2 3 4 5 6 퐹7 (푥) = 1 + 3푥 + 6푥 + 4푥 + 5푥 + 푥 + 푥 1 3 6 4 5 1 1 (2) 2 3 4 5 6 7 퐹8 (푥) = 1 + 4푥 + 6푥 + 10푥 + 5푥 + 6푥 + 푥 + 푥 1 4 6 10 5 6 1 1 (2) 2 3 4 5 6 7 8 퐹9 (푥) = 1 + 4푥 + 10푥 + 10푥 + 15푥 + 6푥 + 7푥 + 푥 + 푥 1 4 10 10 15 6 7 1 1 . . . . Table 2. The first few 2-Fibonacci polynomials and the arrays of their coefficients

Considering the rule of formation of the 2-Fibonacci polynomials, we write the polynomials (2) in ascending order. In order to obtain the coefficient of the 푘-th term of 퐹푛 (푥), we add the (2) (2) coefficients of the (푘 − 1)-st term of 퐹푛−1(푥) and 푘-th term of 퐹푛−2(푥). The array of formed coefficients has the same rule of formation as Pascal’s triangle when it is written in left-justified form, except that each line is written two times and each column is moved one line upper, if we use the formation of the 2-Fibonacci polynomials and write the polynomials in ascending order. Therefore, the coefficients appear along the diagonals formed by beginning in the right-most column and preceding up one and left one throughout the left-justified Pascal triangle whose each line is written two times, e.g., fourth and fifth lines of Pascal’s triangle are written bold.

50 Theorem 3.2. The 2-Fibonacci polynomials are given by

(2) 퐹푛 (푥) = 퐹푛−1 (푥) + 퐹푛 (푥) , 푛 ≥ 1.

Proof. For 푛 = 1, the claim is true, since

(2) 퐹1 (푥) = 1 = 퐹0 (푥) + 퐹1 (푥) .

Suppose that it is true for 푛 = 푘, that is

(2) 퐹푘 (푥) = 퐹푘−1 (푥) + 퐹푘 (푥) .

We should show that the claim is true for 푛 = 푘 + 1. We have

(2) (2) (2) 퐹푘+1 (푥) = 푥퐹푘 (푥) + 퐹푘−1 (푥)

= 푥 (퐹푘−1 (푥) + 퐹푘 (푥)) + 퐹푘−2 (푥) + 퐹푘−1 (푥)

= 푥퐹푘−1 (푥) + 퐹푘−2 (푥) + 푥퐹푘 (푥) 퐹푘−1 (푥)

= 퐹푘 (푥) + 퐹푘+1 (푥) .

We find Binet’s formulas for the 2-Fibonacci polynomials. Let 훼(푥) and 훽(푥) be the solutions of the quadratic equation 푡2 − 푥푡 − 1 = 0: √ √ 푥 + 푥2 + 4 푥 − 푥2 + 4 훼 (푥) = and 훽 (푥) = 2 2 √ √ Notice that 훼(1) = 훼 and 훽(1) = 훽; 훼(2) = 1 + 2 and 훽(2) = 1 − 2 are roots of the characteristic equation 푥2 − 2푥 − 1 = 0 of the Pell recurrence relation. We can verify that 훼푛 (푥)(훼 (푥) + 1) − 훽푛 (푥)(훽 (푥) + 1) 퐹 (2) (푥) = . 푛+1 훼 (푥) − 훽 (푥) (︃ )︃ 푥 1 The 2-Fibonacci polynomials are obtained by the matrix 푄 = , 2 1 0

(︃ )︃푛−1 (︃ )︃푛−2 푥 1 푥 1 푄푛−1 + 푄푛−2 = + 2 2 1 0 1 0

(︃ (2) (2) )︃ 퐹푛 (푥) 퐹푛−1 (푥) = (2) (2) , 퐹푛−1 (푥) 퐹푛−2 (푥) can be shown by mathematical induction. We can write the following interesting determinant identity

(︃ (2) (2) )︃ 퐹푛 (푥) 퐹푛−1 (푥) 푛 det (2) (2) = (−1) 푥. 퐹푛−1 (푥) 퐹푛−2 (푥) Theorem 3.3. 푛 (2) ∑︁ 퐹푛+1 (푥) = 1 + 푥 퐹푖 (푥) . 푖=1

51 Proof. Using the recurrence relation 퐹푛 (푥) = 푥퐹푛−1 (푥) + 퐹푛−2 (푥),

푛 푛 푛 ∑︁ ∑︁ ∑︁ 퐹푖+1 (푥) = 푥 퐹푖 (푥) + 퐹푖−1 (푥) . 1 1 1 That is

(2) 퐹푛+1 (푥) = 퐹푛+1 (푥) + 퐹푛 (푥) 푛 ∑︁ = 푥 퐹푖 (푥) + 퐹0 (푥) + 퐹1 (푥) . 1

Since 퐹0 (푥) = 0, it follows that

푛 ∑︁ 푥 퐹푖 (푥) = 퐹푛+1 (푥) + 퐹푛 (푥) − 1 1 푛 (2) ∑︁ 퐹푛+1 (푥) = 1 + 푥 퐹푖 (푥) . 1 Theorem 3.4. More generally, the sum of the elements along the diagonal beginning at row 푛 is (2) 퐹푛 (푥); that is,

푛−1 [ 2 ] ∑︁ (︂푛 − 푗 − 1)︂ (︂푛 − 푗 − 2)︂ 퐹 (2) (푥) = 푥푛−2푗−1 + 푥푛−2푗−2. 푛 푗 푗 푗=0

Proof. We have ∞ 푦 ∑︁ = 퐹 (푥)푦푛. 1 − 푥푦 − 푦2 푛 푛=0 But 푛−1 ∞ ⎡[ ] ⎤ 1 ∑︁ ∑︁2 (︂푛 − 푗)︂ = (−1)푗 (2푡)푛−2푗 푧푛 1 − 2푡푧 + 푧2 ⎣ 푗 ⎦ 푛=0 푗=0

[ 푛−1 ] ∑︁2 (︂푛 − 푗)︂ 푈 (푡) = (−1)푗 (2푡)푛−2푗. 푛 푗 푗=0 푥 2 Let 푧 = 푖푦 and 푡 = 2푖 , where 푖 = −1, then

∞ 1 ∑︁ 푥 = 푖푛푈 ( )푦푛. 1 − 푥푦 − 푦2 푛 2푖 푛=0 Multiplying the last equation by 푦, we get

∞ 푦 ∑︁ 푥 = 푖푛푈 ( )푦푛+1. 1 − 푥푦 − 푦2 푛 2푖 푛=0

52 It follows that

푛 퐹푛 (푥) = 푖 푈푛 (푥/2푖) 푛 [ 2 ] (︂ )︂ ∑︁ 푗 푛 − 푗 − 1 푛−2푗−1 = 푖푛 (−1) (푥/푖) 푗 푗=0

푛 [ 2 ] ∑︁ (︂푛 − 푗 − 1)︂ = 푥푛−2푗−1. 푗 푗=0

푛−1 [ 2 ] ∑︁ (︂푛 − 푗 − 2)︂ 퐹 (푥) = 푥푛−2푗−2. 푛−1 푗 푗=0

(2) 퐹푛 (푥) = 퐹푛 (푥) + 퐹푛−1 (푥) [ 푛−1 ] ∑︁2 (︂푛 − 푗 − 1)︂ (︂푛 − 푗 − 2)︂ = 푥푛−2푗−1 + 푥푛−2푗−2. 푗 푗 푗=0

(2) Theorem 3.5. The derivative of the 2-Fibonacci polynomial 퐹푛 (푥) is

푛−1 (︀ (2) )︀′ ∑︁ 퐹푛 (푥) = 퐹푖(푥).(퐹푛−푖(푥) + 퐹푛−푖−1(푥)). 푖=1 Proof. It is known that (2) 퐹푛 (푥) = 퐹푛(푥) + 퐹푛−1(푥) (︀ (2) )︀′ ′ 퐹푛 (푥) = (퐹푛(푥) + 퐹푛−1(푥)) 푛 ′ ∑︁ (퐹푛(푥)) = 퐹푖 (푥) 퐹푛−푖 (푥) 푖=1 푛−1 ′ ∑︁ (퐹푛−1(푥)) = 퐹푖 (푥) 퐹푛−푖−1 (푥) 푖=1 푛−1 (︀ (2) )︀′ ∑︁ 퐹푛 (푥) = 퐹푖 (푥)(퐹푛−푖 (푥) + 퐹푛−푖−1 (푥)) . 푖=1 Example 3.6. The derivative of

(2) 4 3 2 퐹5 (푥) = 푥 + 푥 + 3푥 + 2푥 + 1 is 4 ∑︁ 퐹푖 (푥)[퐹5−푖 (푥) + 퐹5−푖−1 (푥)] = 퐹1 (푥)[퐹4 (푥) + 퐹3 (푥)] 푖=1

+ 퐹2 (푥)[퐹3 (푥) + 퐹2 (푥)]

+ 퐹3 (푥)[퐹2 (푥) + 퐹1 (푥)]

+ 퐹4 (푥)[퐹1 (푥) + 퐹0 (푥)]

53 = 퐹1 (푥) 퐹4 (푥) + 퐹1 (푥) 퐹3 (푥)

+ 퐹2 (푥) 퐹3 (푥) + 퐹2 (푥) 퐹2 (푥)

+ 퐹3 (푥) 퐹2 (푥) + 퐹3 (푥) 퐹1 (푥)

+ 퐹4 (푥) 퐹1 (푥) + 퐹4 (푥) 퐹0 (푥)

= 2퐹1 (푥) 퐹4 (푥) + 2퐹1 (푥) 퐹3 (푥) 2 + 2퐹2 (푥) 퐹3 (푥) + 퐹2 (푥) = 2 · 1 · (︀푥3 + 2푥)︀ + 2 · 1 · (︀푥2 + 1)︀ + 2 · 푥 · (︀푥2 + 1)︀ + 푥2 = 4푥3 + 3푥2 + 6푥 + 2 .

4 Conclusion

In the present paper, we define 2-Fibonacci polynomials and give some identities between the 2-Fibonacci polynomials and Fibonacci polynomials. Also, we present some properties of the 2-Fibonacci polynomials. We introduce the derivative of the derivatives of the 2-Fibonacci poly- nomials. This enables us to give in a straightforward way several formulas for the sums of such polynomials. These identities can be used to develop new identities of polynomials.

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