Abstract
We consider linear recursions of length two and related gap recursions where the indices may not be consecutive integers. Given a linear recursion of length two, we prove the existence of an explicit linear recursion of length with arbitrary distance between indices. Conversely, it is shown under some mild assumption that a linear recursion of length can be reduced to one of length two.
1. Introduction
The sequence of Fibonacci numbers Fn is defined by the recursion for integer starting with the initial values and Livio (Citation2002) provided an explicit formula for the nth Fibonacci number, i.e. (1) (1) where is the golden ratio and It is an easy exercise to prove this formula by induction or using the associated generating function.
Fibonacci numbers appear in various ways in different areas. In botany, phyllotaxis is the study of the arrangement of leaves on a plant stem. In 1830, Schimper (Citation1830) observed that the ratio of the number of circuits divided by the number of leaves—the so-called divergence—in the spiral configurations often result as quotients of Fibonacci numbers with gap; for example, the divergence is 1/3 for hazel, 2/5 for apricot, 3/8 for pear, and 5/13 for almond (cf (Hellwig & Neukirchner, Citation2010)). The sequence of rational numbers is called the Braun-Schimper sequence (named after Alexander Braun a colleague of Schimper). In 2015 Wang, Li, Kong, and He (Citation2015) discovered that a fractal dimension of polar bear hairs is close to the golden ratio 1.618…. Sinha (Citation2017) showed amazing applications of Fibonacci numbers in nature including constructing security coding. Blankenship (Citation2021) linked the Fibonacci sequence to music and Guo (Citation2021) used the Fibonacci sequence to make predictions on the Parkfield earthquake sequence. Recently, Postavaru and Toma (Citation2022) studied the time scale Fibonacci sequences satisfying the Friedmann-Lemaître-Robertson-Walker (FLRW) dynamic equation on time scale, which is an exact solution of Einstein’s field equations of general relativity for expanding homogeneous and isotropic universe. They investigated the results based on the dynamic equations on time scale describes the Fibonacci numbers. In the same year, Basak (Citation2022) proposed to construct a philosophical theory based on unexplained natural phenomena, observing the golden ratio and Fibonacci sequence in art and nature such as plants, animals, storm and universe. He also discussed these phenomena within his philosophical aspects based on the literature evidences.
Since Fibonacci sequence is in the form of a linear recursion of length two, it inspires us to consider any arbitrary recursions of length two and higher length. It may come as a surprise that any number of further recursions for Fibonacci numbers can be formed with arbitrary distances between the indices. For example, for every pair of positive integers k and there exist coefficients depending only on such that (2) (2) for every we shall call this a gap recursion. In fact, the equations for and determine γ and δ and the verification of the gap recursion simply follows from the classical recursion formula by induction. This yields (3) (3) or (4) (4) for arbitrary positive integers
In this short note, we study the more general situation of a sequence of real numbers Xn satisfying a recursion of the form (5) (5) where α, β are again fixed real numbers. Our aims are to answer the following questions:
For every positive integers and distinct do there exist coefficients such that (6) (6) And, in case of their existence, is there a corresponding explicit formula for Xn in terms of the gap recursion?
Conversely, let for with a positive integer and real numbers γi. Are there two real numbers α, β such that (7) (7)
for every integer ?
We give the answer to the first and the second question in Sections 2 and 3, respectively by applying fundamental techniques from linear algebra and elementary calculations with complex numbers.
2. Extension of the recursion of length two
Given any sequence of real numbers Xn satisfying the following recursion (8) (8) it follows that, for any positive integer we have (9) (9) where with and with Hence, for any positive integers and non-negative integers where for all j, for and for we obtain (10) (10) (11) (11)
If it is easy to see that (12) (12)
If and we derive (13) (13)
This implies (14) (14) where (15) (15)
If and we obtain similarly (16) (16)
This implies (17) (17) where (18) (18)
This leads to
Theorem 1.
Given real numbers α, β, let . Define
with and
with
Then, for every pair of positive integers and non-negative integers , where for all , let such that for and for . Then (19) (19)
where (20) (20) (21) (21)
and (22) (22)
3. Reduction of the recursion of length
We consider the recursion (23) (23) where is a positive integer and the γi’s are real numbers such that Recalling the question from the introduction, is there a relation for some real numbers α, β?
In 2017, Wiboonton (Citation2017) collected several methods for research on the Fibonacci sequence. One of them relies on matrix calculus. We shall follow this approach and consider: (24) (24)
This implies, for any positive integer k, (25) (25)
In the sequel we assume that α and β satisfy (in order to have a non-vanishing discriminant). We can diagonalize by an invertible matrix and a diagonal matrix such that (26) (26) where and
If we set then we can rewrite relation Equation(23)(23) (23) as (27) (27)
Applying the above matrices, we obtain (28) (28) or (29) (29)
Now, we set This implies Hence, we have (30) (30)
Then, (31) (31)
To solve this system, we assume that there are two complex numbers z1 and z2 satisfying If we set and then we get and In case that z1 and z2 are real numbers, both α and β are also real, however, it remains to verify whether is non-vanishing or not.
For otherwise, we choose which is a root of and its conjugate is also a root. In this case the values α and β are and respectively. Furthermore, since z is not a real number.
Summing up, we arrive
Theorem 2.
Let be a positive integer and be a linear homogeneous recursion with initial conditions Xi for and suppose that the sequence of Xn satisfies at least one of the following statements:
the corresponding characteristic equation has two real roots r1 and r2 such that and the quadratic recursion (32) (32) with the initial conditions of Y 0 = X0 and Y 1 = X1 leads to Y i = Xi for all
its characteristic equation has a complex root z such that z is not a real number and the quadratic recursion (33) (33)
with the initial conditions of Y 0 = X0 and Y 1 = X1 leads to Y i = Xi for all
Then, Y n = Xn for all n.
Applying this to the second question from the beginning, it follows that there always exist two real numbers α, β satisfying whenever the associated characteristic equation satisfies at least one of the two conditions in Theorem 2.
4. Applications and examples
To illustrate the theorems from above, we shall consider the well-known recursion for Fibonacci number Fn.
Example 3.
It is easy to see that if the indices are restricted to the set of non-negative odd or even integer, there are also recursions, namely (34) (34)
Indeed, we may apply Theorem 1 with and Bn = Fn for any positive integer n. We deduce (35) (35) or (36) (36) for any positive integers (that are formulae (Guo, Citation2021) and (Hellwig & Neukirchner, Citation2010) from the introduction). For the special case , we get (37) (37) or (38) (38)
(which is formula Equation(34)(34) (34) above).
On the other hand, by Theorem 2, the corresponding characteristic equation of the recursion is which has four distinct real roots, namely (39) (39)
If the initial values are , we get that
If the initial conditions are , we get that
5. Conclusions
From Sections 2 and 3, we conclude the main results as follows:
Remark 4
In Theorem 1 (Section 2), we extend the length of a linear recursion of length two to one of length . Conversely, we can reduce the length of a linear recursion of length to one of length two in Theorem 2 (Section 3).
Remark 5
We observe that a linear recursion of length that is constructed from Theorem 1 (Section 2) can always be reduced to linear recursion of length two by using the method in Theorem 2 (Section 3). However, for any linear recursion of length , if it is written in any form of linear recursions of length two obtained by Theorem 2 but there is no initial condition of each recursion of length two which corresponds to one of length , we cannot conclude that there is not a linear recursion of length two which represents the same sequence.
Acknowledgments
The authors are grateful to the anonymous referee for her or his valuable comments and corrections. The authors would like to thank National Research Council of Thailand (NRCT) and Walailak University for facilities and financial supports.
Disclosure statement
No potential conflict of interest was reported by the authors.
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References
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