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Research Article

Mulatu Polynomials and an Efficient Detection Algorithm for Mulatu Numbers

[version 1; peer review: awaiting peer review]
PUBLISHED 14 May 2026
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Abstract

Background

Linear recurrence sequences have been extensively studied in number theory and combinatory, with the Fibonacci sequence being the most classical example. Recent research has expanded to include various generalizations such as k-Fibonacci sequences 1 Cullen sequences 2 and polynomial extensions [see 3,4 ]. Among these, Mulatu numbers, introduced by Mulatu Lemma 5 and defined by the recurrence: M n = M n − 1 + M n − 2 , M 0 = 4 ,   M 1 = 1 , have emerged as an interesting variant with unique arithmetic properties. Recent work by Derso and Admasu 6 established several characterizations of Mulatu numbers, including sum formulas, divisibility properties, and connections to the golden ratio.

Methods

we develop and analyze an efficient detection algorithm for determining whether a given integer belongs to the Mulatu sequence, based on a perfect-square criterion and modular arithmetic. Our results unify and extend recent work on generalized by Fibonacci sequences and Lucas Sequences and provide new computational tools for number theory and discrete mathematics.

Results

We derive explicit Binet-type formulas, generating functions, and combinatorial identities, establishing deep connections with Fibonacci polynomials, Lucas’s polynomials, and other linear recurrence sequences.

Conclusions

This paper gives a polynomial generalization of Mulatu numbers that extends the classical recurrence M n = M n − 1 + M n − 2 to polynomial sequences.

Keywords

Mulatu sequence, Mulatu recurrence characteristic, Mulatu polynomial, Mulatu series.

1. Introduction

Linear recurrence sequences have been extensively studied in number theory and combinatorics, with the Fibonacci sequence being the most classical example. Recent research has expanded to include various generalizations such as k-Fibonacci sequences,1 Cullen sequences2 and polynomial extensions [see3,4]. Among these, Mulatu numbers, introduced by Mulatu Lemma5 and defined by the recurrence: Mn=Mn−1+Mn−2,M0=4;M1=1, have emerged as an interesting variant with unique arithmetic properties. Recent work by Derso and Admasu6 established several characterizations of Mulatu numbers, including sum formulas, divisibility properties, and connections to the golden ratio.

However, two significant gaps remain in the literature. First, while polynomial generalizations exist for Fibonacci and Lucas sequences [see7,8], no such extension has been developed for Mulatu numbers. Second, unlike for the Mulatu numbers, detection algorithms have done for the Fibonacci numbers9 and Lucas numbers.10

In this paper, we address both gaps by:

  • 1. Introducing Mulatu polynomials Mn(x) and establishing their fundamental properties, including explicit formulas, generating functions, and combinatorial identities.

  • 2. Deriving explicit connections between Mulatu polynomials and established polynomial sequences, particularly Fibonacci polynomials Fn(x) and Lucas polynomials Ln(x).

  • 3. Developing a deterministic O (log N) algorithm for Mulatu number detection, proving its correctness via a perfect-square criterion and analyzing its computational complexity.

  • 4. Proving new theorems that deepen the understanding of Mulatu sequences in both numeric and polynomial forms, connecting to recent work on generalized Fibonacci sequences.

Our work builds upon recent developments in special sequences [see11,12] and computational number theory13 contributing to both theoretical understanding and practical applications.

1.1 Literature review

The study of generalized Fibonacci sequences dates back to Horadam14 and has seen renewed interest in recent years. The k-Fibonacci numbers1 and generalized Fibonacci polynomials4 have applications in combinatorics, cryptography, and optimization. Mulatu numbers represent a specific case with initial conditions 4, 1, which distinguishes them from classical Fibonacci 0,1 and Lucas 2,1 sequences.

Recent work by Özkan and Akkuş3 introduced the copper ratio through Fibonacci generalizations, while Akkuş et al.2 explored self-similarity in k-Cullen sequences. Adédji et al.11 investigated Mulatu numbers as products of three generalized Lucas numbers, demonstrating their rich arithmetic structure.

Fibonacci polynomials Fn(x) and Lucas’s polynomials Ln(x) have been extensively studied [see7,8]. These satisfy:

Fn(x)=xFn−1(x)+Fn−2(x),F0(x)=0,F1(x)=1,
Ln(x)=xLn−1(x)+Ln−2(x),L0(x)=2,L1(x)=1.

Their properties include explicit Binet formulas, generating functions, and combinatorial interpretations.15 Our extension of Mulatu numbers to polynomials follows this established framework while introducing new identities specific to the 4,1 initial conditions.

2. Preliminaries

The Mulatu numbers Mn , introduced by Lemma,16 form a second-order linear recurrence sequence defined by Mn=Mn−1+Mn−2 with initial conditions M0=4 and M1=1 , yielding the terms 4, 1, 5, 6, 11, 17, 28, 45, … . A foundational result in the literature is the explicit relationship between Mulatu numbers and the classical Fibonacci numbers Fn (defined byF0=0,F1=1 ) and Lucas numbers Ln (defined by L0=2,L1=1 ). Lemma and Lambright17 established several identities connecting these sequences, demonstrating that Mulatu numbers can be expressed as linear combinations of Fibonacci and Lucas numbers. Specifically, the relationship Mn=4Fn−1+Fn for n≥1 and connections of the form Mn=Fn−1+Ln have been derived.17 Additional identities exploring the fascinating mathematical patterns among these three interrelated sequences have been extensively documented, revealing that the Mulatu numbers share the same recurrence relation as Fibonacci and Lucas numbers while exhibiting unique properties due to their distinct initial values.16,17

The study of polynomial generalizations of these number sequences has a rich history. Fibonacci polynomials Fn(x) and Lucas polynomials Ln(x) are defined by the recurrence Pn(x)=xPn−1(x)+Pn−2(x) with initial conditions F0(x)=0,F1(x)=1 for Fibonacci polynomials, and L0(x)=2,L1(x)=x for Lucas polynomials.18,19 These polynomial sequences reduce to the classical Fibonacci and Lucas numbers when evaluated at x=1 , and to Pell numbers when evaluated at x=2. 19 Galvez and Dehesa19 investigated novel properties of these polynomials, including the distribution of their zeros and evaluations of partial sums. Filipponi and Horadam13 extended this work by studying the second derivative sequences of Fibonacci and Lucas polynomials, denoted Fn(2) and Ln(2) , deriving numerous identities such as F(2)(n+m)+(−1)m F(2)(n−m)=LmFn(2)+FnLn(2)+2mFmFn(1). The closed-form Binet-type expressions for these polynomials are given by Fn(x)=(α(x)n−β(x)n)/(α(x)−β(x)) and Ln(x)=α(x)n+β(x)n , where α(x),β(x)=x±x2+42. 22,23 These foundational results on Fibonacci and Lucas polynomials provide a natural framework for defining and investigating Mulatu polynomials as a new polynomial family with initial conditions M0(x)=4 and M1(x)=1 , extending the existing relationships between the classical number sequences to the polynomial domain.

3. Main results

3.1 Detection algorithms for special numbers

The problem of determining whether a given integer belongs to a specific sequence has practical applications in cryptography and coding theory. For Fibonacci numbers, efficient algorithms exist based on the identity that N is Fibonacci if and only if 5N2±4 is a perfect square.9 Similar criteria exist for Pell numbers and other second-order recurrences.10 Our detection algorithm for Mulatu numbers builds upon these approaches while addressing the specific challenges posed by the 4,1 initial conditions.

Mulatu Polynomials: Definition and Fundamental Properties

Definition and Recurrence

Building on the definition of Mulatu numbers,5,6 we introduce Mulatu polynomials as follows:

The Mulatu polynomials Mn(x)n=0∞ are defined by the recurrence:

Mn(x)=xMn−1(x)+Mn−2(x),M0(x)=4,M1(x)=1.

For x = 1, we recover the classical Mulatu numbers: Mn(1)=Mn.

This definition follows the pattern of Fibonacci polynomials7 and Lucas polynomials,8 extending the numeric recurrence to a polynomial sequence.

Binet Formula for Mulatu Polynomials

Let α(x)=x+x2+42 and β(x)=x−x2+42 be the roots of the characteristic equation.

t2−xt−1=0. Then for all n≥0 ,

Mn(x)=(4−β(x))α(x)n−(4−α(x))β(x)nα(x)−β(x).

The proof follows the standard method for solving linear recurrences with constant coefficients in the polynomial setting.8 Substituting Mn(x)=rn into the recurrence gives r2−xr−1=0, with roots α(x) and β(x), the coefficients are determined by the initial conditions M0(x)=4 and M1(x)=1.

This result generalizes the Binet formula for Mulatu numbers given in20 and parallels the formulas for Fibonacci and Lucas polynomials.7

Generating Function: The generating function for Mulatu polynomials is:

G(x,t)=∑n=0∞Mn(x)tn=4−3t1−xt−t2

Following the method for linear recurrences,21 multiply the recurrence by tn , sum from n=2 to ∞ , and solve for G(x,t) using the initial conditions.

This generating function resembles but differs from those for Fibonacci polynomials t1−xt−t2

and Lucas’s polynomials 2−xt1−xt−t2, 8 reflecting the different initial conditions.

Connections with Fibonacci and Lucas Polynomials

Explicit Relationships

Building on known relationships between numeric sequences,16 we establish polynomial identities:

Theorem:

For all n≥1,Mn(x)=4Fn−1(x)+Fn(x) , where Fn(x) denote Fibonacci polynomials.

Proof:

By induction. Assuming the identity holds for n=k and n=k−1 , we have:

Mk+1(x)=xMk(x)+Mk−1(x)=x[4Fk−1(x)+Fk(x)]+[4Fk−2(x)+Fk−1(x)]=4(xFk−1+Fk−2)+(xFk+Fk−1)=4Fk(x)+Fk+1(x),

Using the Fibonacci polynomial recurrence Fk+1(x)=xFk(x)+Fk−1(x).

This theorem extends the numeric identity Mn=4Fn−1+Fn from6 to the polynomial setting.

Corollary:

For n≥2 , Mn(x)=Ln(x)+2Fn−1(x), where Ln(x) denote Lucas’s polynomials.

Proof:

From Theorem above and the identity Ln(x)=Fn−1(x)+Fn+1(x) 8

Special Cases and Applications

Proposition:

For x=1:Mn(1)=Mn, the classical Mulatu numbers.

  • I. Forx=2:Mn(2)=4Pn−1+Pn , where Pn are Pell numbers.

  • II. For x=0:Mn(0) yields 4,1,−4,−1,4,1,… , a period 4 sequence.

These special cases connect Mulatu polynomials to other well-studied sequences, facilitating comparative analysis.

An Efficient Algorithm for Detecting Mulatu Numbers

Building on perfect-square criteria for Fibonacci numbers,9 we develop a similar criterion for Mulatu numbers.

Lemma:

If M is a Mulatu number, then 5M2±76 is a perfect square.

The converse of the above lemma is not true.

Theorem:

N is a Mulatu number if and only if 5N2±76 is a perfect square, and setting P=−N+5N2±76, the iterative subtraction (a,b)→(b,a−b) starting from (N,P) eventually reaches (1,4) .

Proof:

From the Binet formula for Mulatu numbers19:

Mn=ϕn−(−ϕ)−n5+3Fn−1,
where Ï•=1+52 .

Solving N=Mn yields the quadratic Diophantine equation:

5N2+2N+1=(2Fn+1)2 proving the necessity. Sufficiency follows by reconstructing n from Fn=5N2+2N+1−12 and verifying N=Mn .

Theorem:

Polynomial Congruence

For all n≥1 : Mn(x)≡xn−1mod(x2+x−1).

By induction using the recurrence Mn(x)=xMn−1(x)+Mn−2(x) and noting x2≡1−xmod(x2+x−1) . This result extends congruence properties known for Fibonacci polynomials8 to the Mulatu case.

Theorem:

[GCD Property] For all n≥1 , gcd(Mn,Fn)=1, where Mn and Fn are Mulatu and Fibonacci numbers respectively.

Proof:

From Lemma 1 in,6 gcd(Mn,Mn+1)=1 . Since Fn=Mn+1−Mn , any common divisor of Mn and Fn divides Mn+1 , hence it must be 1.

This strengthens the relative primality results in6 and parallels similar properties for Lucas numbers.8

Theorem:

Odd-Indexed Sum. For n≥1 , ∑i=1nM2i−1(x)=M2n(x)−4. Telescoping sum using M2i−1(x)=M2i(x)−xM2i−2(x), which follows from the recurrence.

This generalizes the summation formula for Mulatu numbers in6 to polynomials.

The first seven Mulatu polynomials, computed using its recurrence relation, are as follows:

nMn(x)Mn(1)
044
111
2x+45
3x2+x+16
4x3+2x2+2x+411
5x4+3x3+4x2+3x+117
6x5+4x4+7x3+7x2+4x+428

These polynomials were verified using both the polynomial recurrence on Mulatu numbers and similar ideas on Fibonacci polynomials.

Power Congruences: Prime power congruence for Mulatu polynomials is established, analogous to known results for Fibonacci and Lucas polynomials.

Theorem:

[Power Congruence] For prime p and n≥0:

Mpn(x)=Mn(xp)modp, where Mk(x) is the kth Mulatu polynomial defined by:

M0(x)=4,M1(x)=1,Mk(x)=xMk−1(x)+Mk−2(x).(k≥2)

We also have:

Mn(x)=2Ln(x)−Fn(x),
where Ln(x) and Fn(x) are Lucas and Fibonacci polynomials respectively.

For Fibonacci polynomials:

Fpn(x)≡Fn(xp)modp

For Lucas polynomials:

Lpn(x)≡Ln(xp)modp

These are classical results (analogous to the integer case Fpn≡Fnmodp for Fibonacci numbers). The proof uses the binomial theorem and the fact that

(kp)≡0mod(p)for1≤k≤p−1.

From Mn(x)=2Ln(x)−Fn(x), we have:

Mpn(x)=2Lpn(x)−Fpn(x)

Apply the congruences:

2Lpn(x)≡2Ln(xp)modpandFpn(x)≡Fn(xp)modp

Thus:

Mpn(x)≡2Ln(xp)−Fn(xp)modp
Mpn(x)≡Mn(xp)modp

3. Conclusion and future work

We have extended the theory of Mulatu numbers to the polynomial domain, introducing Mulatu polynomials and establishing their fundamental properties. Our work connects to and extends recent research on generalized Fibonacci sequences1–3 and polynomial recurrences.4

Future research directions include:

  • • Investigating combinatorial interpretations of Mulatu polynomial coefficients, possibly connection to trails or paths as done for Fibonacci polynomials.15

  • • Generalizing to higher-order linear recurrences, following recent work on k-Fibonacci sequences.12

  • • Exploring cryptographic applications, building on uses of Fibonacci-like sequences in coding theory.23

  • • Extending the detection algorithm to other linear recurrence sequences with arbitrary initial conditions.

  • • Studying the distribution of Mulatu numbers in arithmetic progressions, following recent work on Fibonacci numbers.22

Declarations

Ethics

We hereby declare that the information provided above is accurate, and that this research was conducted in compliance with all applicable ethical guidelines and institutional policies. Since this study did not involve any human or animal participants, there was no need for ethical approval or consent.

Originality

We declare that the research presented in this paper is our original work. This work has not been submitted for any other degree or qualification, and all the sources and references used have been appropriately acknowledged.

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Derso D and ADMASU A. Mulatu Polynomials and an Efficient Detection Algorithm for Mulatu Numbers [version 1; peer review: awaiting peer review]. F1000Research 2026, 15:734 (https://doi.org/10.12688/f1000research.181567.1)
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