A New Fractional Gradient Representation of Birkhoff Systems

A fractional generalization of the gradient system to the Birkhomechanics, that is, a new fractional gradient representation of the Birkho system is investigated, in this paper. e denitions of the fractional gradient system are generalized to Birkho mechanics. Based on the denition, a general condition that a Birkho system can be a fractional gradient system is derived, the former studies for fractional gradient representation of the Birkho system are special cases of this paper. As applications of the results, the Birkho equations and fractional gradient expression of several classical nonlinear models are derived, such as the Hénon–Heiles equation, the Dung oscillator model, and the Hojman–Urrutia equations.e results indicate, dierent from the former studies, that only gave the second-order gradient (integer order) expression for the Birkho system, an arbitrary fractional order gradient representation, exists for the Birkho system.e fractional potential function obtained from the general condition can determine the stationary states of these models in Birkhoan expression.


Introduction
Fractional integrals and derivatives are increasingly important in the modeling of science and engineering problems because they are more suitable for describing complex phenomena in science and engineering. e discussion of noninteger order derivatives dates back to Leibniz and L'Hopital in 1695, but until 1974 the rst monography [1] written by Oldham and Spanier about fractional integrals and derivatives was published. Since then, fractional calculus has been applied in many elds such as physics, mechanics, etc. [2][3][4][5][6][7][8][9][10][11][12][13][14][15].
A gradient system is an important dynamical system. e characteristics of a gradient system are suitable, especially to determine the stability of a system by using the Lyapunov function [16][17][18][19][20]. If a mechanical system can be transformed into a gradient system, the stability of the mechanical system can be studied through the characteristics of the gradient system. Many researchers have performed the gradient representations of constrained mechanical systems such as the Lagrange system, the Hamilton system, and the Birkho system and studied the stability of their solutions [21][22][23][24][25][26][27]. Tarasov [28][29][30] generalized the Hamilton system and the Lagrange system into fractional gradient expressions. Mei et al. [31,32] studied the fractional gradient representation of the Birkho system, Chen et al. [33] gave a fractional gradient representation of Poincaré equations. However, their studies only provide a special condition with second order (α 2) and only can provide a second order gradient system, which limits its application in constrained mechanical systems. In this paper, our goal is to generalize the fractional gradient representation of the Birkho system to any order.
e Birkho system is an important dynamical system; it is an extension of the Hamilton system. Birkho mechanics has important applications in hadronic physics, statistic mechanics, space mechanics, biophysics, and engineering [34]. Birkho 's mechanical system is a more general, constrained mechanical system [35][36][37]. Conservative and nonconservative systems and holonomic and nonholonomic systems can all be expressed by Birkho an formulations, which have a generalized symplectic structure [38]. So, generalizing the fractional gradient system into the Birkho system has general signi cance. Since former studies about the fractional gradient representation of the Birkho system only gave its second-order expression. In this paper, we will show the order of the fractional gradient Birkho system is not only with α 2 but also can be other orders (integer or noninteger order), and give a general condition for the Birkhoff system transforming into a fractional gradient Birkhoff system. e structure of this paper is as follows: In Section 2, we will fix the notations of fractional derivative and fractional exterior derivative and give a brief review of the definition of the fractional gradient system and its formulation. In Section 3, we will discuss under what conditions a Birkhoff system can transform into a fractional gradient system. In Section 4, we will apply the results that we got in Section 3 to several examples, such as the Hénon-Heiles equation, the Duffing oscillator model, and the Hojman-Urrutia equations. Section 5 is the conclusion.

Review of the Fractional Gradient System
Fractional derivatives have different definitions. In this paper, we will adopt the Caputo and Riemann-Liouville fractional derivative definitions. Based on the definition of the Riemann-Liouville fractional derivative, we have the definition of D α x in the Riemann-Liouville fractional derivative sense.
where m is the first whole number greater than α > 0. e initial point of a fractional derivative is set to be zero. e Riemann-Liouville fractional derivative has some disadvantages in physical applications, such as the hyper-singular improper integral, where the order of singularity is higher than the dimension, and the nonzero of the fractional derivative of constants, which would make dissipation not vanish for a system in equilibrium. e Caputo fractional derivatives can choose the same initial value as the integer derivatives, which are more suitable for applying to physical problems. e Caputo fractional derivative has the following definition: where When 0 < α ≤ 1, a � 0, and m � 1, we have We can introduce fractional exterior derivatives as [29][30][31]39] For the convenience of expression, we use D α x to denote α order fractional derivative, and only point out whether it is the Riemann-Liouville fractional derivative or the Caputo fractional derivative as calculation.
e differential equations of a gradient system have the form as where x � (x 1 , x 2 , . . . , x n ), and V is a potential function. Following, we will review the definition and conditions of the fractional gradient system proposed by Tarasov [28][29][30].
If a fractional differential 1-form, is is an exact form, that is, is a continuous differential function, then system (7) is a fractional gradient system. us, we have It is known that to be exact, it is a sufficient condition to be closed; however, the converse statement is not necessarily true. From the Poincaré theorem that is verified to be true for fractional exterior derivatives, it can be deduced that any smooth 1-form (8) that is closed is also exact on a contractible open subset W of R n [28]. We have the following proposition.

Proposition 1. If smooth functions
then dynamical system (7) is a fractional gradient system with form where V � V(x) is a continuous differential function and . Because the Riemann-Liouville fractional derivative of a constant is not necessary to be zero, so, V(x) � C cannot define the stationary state of a fractional system (10), where C is constant. e stationary state of the fractional gradient system with the Riemann-Liouville fractional derivative can be defined by where C k1···k n are constants and m is the first whole number greater than or equal to α. If the fractional exact 1-form 8 equals to zero that is d α V � 0 in the Caputo derivative, we can get V(x) − C � 0, which defines the stationary state of the fractional gradient system (10), where C is constant.

Fractional Gradient Representation of the Birkhoff System
An automatic Birkhoff system has form where B � B(a) is a Birkhoffian, and is called the Birkhoffian tensor and R μ (a) are Birkhoff's functions. If the system is nonsingularity, that is det(Ω μ] ) ≠ 0, the automatic Birkhoff system (12) can be expressed as where Definition 2. For a Birkhoff system (12) or (14), if the fractional differential 1-form is an exact form, that is ω α � −d α V, then it is a fractional gradient expression of the Birkhoff system. Make exterior derivative to fractional differential 1-form (16), we have So, we have the following proposition. (14) on a contractible open subset W of R n satisfies condition (18), it is an α order fractional gradient system. Remark 1. When α � 1, condition (18) degenerates to an integer derivative case, which gives the condition for a Birkhoff system transforming into a classical gradient expression [21].

Proposition 2. If equation
When α > 1 (including integer and noninteger numbers), we call it an α order fractional gradient expression of the Birkhoff system.
We point out that 0 < α < 1 cannot be a fractional gradient system for the Birkhoff system, it may be a fractional gradient system for the fractional Birkhoff system, which question we will discuss in future works. From (10), we have According to Birkhoff 1-form (19), we can get the expression of the potential function V(a).
where I α 0 is the fractional Riemann-Liouville integral symbol with definition Remark 3. Here α > 1 can be any order, but we should point out that only (20) makes the Birkhoff 1-form is exact, and (19) is a fractional order gradient expression of the Birkhoff system.

Application A: Hénon-Heiles Equation as a Birkhoff
System and Fractional Gradient System. In this section, we will give the Birkhoff expression of the Hénon-Heiles equation and prove it can be a fractional gradient system with the Riemann-Liouville or the Caputo fractional derivatives, respectively, in the Birkhoff mechanics frame. e Hénon-Heiles equation is a sort of nonlinear, nonintegrable Hamilton system, which has broad applications in physics and applied mathematics [10,40,41]. e Hénon-Heiles equation's form is erefore, we can get Mathematical Problems in Engineering 3 e Birkhoff equations can be expressed as From (16), we can get a fractional differential 1-form of Birkhoff (26).
Its fractional exterior derivative is Next, we will discuss its fractional gradient expression by the Caputo and Riemann-Liouville fractional derivatives, respectively.

Fractional Gradient Representation of the Birkhoff Equation (27) with the Caputo Fractional Derivative.
Firstly, we study its fractional gradient system using the Caputo fractional derivative. As we take the Caputo fractional derivative, that is, So d α ω α ≠ 0, then (26) cannot be expressed as a fractional gradient system. When 1 < α ≤ 2, we can get So d α ω α ≠ 0, then (26) cannot be expressed a fractional gradient system.
When m < α ≤ m + 1 with m ≥ 2, d α ω α � 0, so (26) can be expressed as an α order Caputo type fractional gradient system, where (m < α ≤ m + 1, m ≥ 2). From (19), we can get According to (20), we can get the potential function as where β > 0. We can directly calculate that D α a μ V(a) is not the Birkhoff system (27). In order to make the potential function V (33) is the potential function of the Birkhoff system (27), we have to modify (33) as Here, m is the first integer greater than or equal to α > 0. However, it does not guarantee the system is a fractional gradient system, so considering d α ω α � 0, we have m + 1 ≥ β � α > m ≥ 2, the Birkhoff system (27) can be expressed as an α order Caputo fractional gradient system. e stationary states of the Birkhoff systems are defined by equation 4 Mathematical Problems in Engineering where C is constant.

Fractional Gradient Representation of the Birkhoff Equation (27) with the Riemann-Liouville Fractional
Derivative. Secondly, we study the fractional gradient representation of the Birkhoff system (27) using the Riemann-Liouville fractional derivative. When we take the Riemann-Liouville fractional derivative that is When α � 1, we have (27) is not a gradient system. When α � 2, we have
From (19), we can get e potential function can be written as with β � α.

Conclusion.
In this section, we give the Birkhoff expression of the Hénon-Heiles equation and give its fractional gradient representation using the Caputo fractional derivative and the Riemann-Liouville fractional derivative, respectively. We can conclude that from this example, the Hénon-Heiles equation in the Birkhoff mechanics frame can be expressed as an α (noninteger) order fractional gradient system using the Caputo fractional derivative, or as a thirdorder fractional gradient system using the Riemann-Liouville fractional derivative. However, it cannot be a secondorder gradient system, so our results are more general than those in [31][32][33]. We also conclude that a fractional gradient system is not a classical gradient system in general. From the calculus, we find that using the Riemann-Liouville fractional derivative can only give an integer order gradient system, but using the Caputo fractional derivative can give an integer or noninteger order gradient system, so the Caputo fractional derivative is more suitable to study fractional gradient systems than the Riemann-Lioville fractional derivative.

Application B: The Birkhoff Equation of Duffing Oscillator Models and Its Fractional Gradient System
In this section, we will give the Birkhoff equation expression of the Duffing oscillator models and prove it can be a fractional gradient system with the Riemann-Liouville or the Caputo fractional derivatives, respectively, in the Birkhoff mechanics frame. Duffing oscillator models and their equation are widely discussed in nonlinear science: where ϵ is the nonlinear stiffness coefficient of the Duffing oscillator. Take Birkhoff's variables as We can get Birkhoff expression of the Duffing oscillator equation Mathematical Problems in Engineering From (16), we can get a fractional differential 1-form of Birkhoff equation (43).
Its fractional exterior derivative is (45) Next, we will discuss its fractional gradient expression using the Caputo and the Riemann-Liouville fractional derivatives, respectively. (43) with the Caputo Fractional Derivative. Firstly, we study its fractional gradient system using the Caputo fractional derivative. As we take the Caputo fractional derivative, that is,

Fractional Gradient Representation of Birkhoff System
en d α ω α ≠ 0, so the Duffing oscillator system expressed by the Birkhoff equation (43) is not a fractional gradient system. When 1 < α ≤ 2, we can get en d α ω α ≠ 0, so the Duffing oscillator system expressed by the Birkhoff equation (43) is not a fractional gradient system. When 2 < α ≤ 3, we can get en d α ω α ≠ 0, so the Duffing oscillator system expressed by the Birkhoff equation (43) is not a fractional gradient system.
(50) e stationary state of system 43 in the Caputo derivative is defined by equation (35).

Conclusion.
In this section, we give the Birkhoff equation expression of the Duffing oscillator problem and give its fractional gradient representation using the Caputo fractional derivative and the Riemann-Liouville fractional derivative, respectively. We conclude that, from this example, the Duffing oscillator model in the Birkhoff mechanics frame can be expressed as a α (noninteger) order fractional gradient system using the Caputo fractional derivative or a fourth order fractional gradient system using the Riemann-Liouville fractional derivative. However, it cannot be a second-order gradient system, so our results are more general than those in [31][32][33]. We also conclude that a fractional gradient system is not a classical gradient system in general. From the calculus, we find that using the Riemann-Liouville fractional derivative can only give an integer order gradient system, but using the Caputo fractional derivative can give an integer or noninteger order gradient system, so the Caputo fractional derivative is more suitable to study fractional gradient systems than the Riemann-Liouville fractional derivative.

Application C: Hojman-Urrutia Equations as a Birkhoff System and Fractional Gradient System
In this section, we will give the Birkhoff equation expression of the Hojman-Urrutia equations, and prove it can be a fractional gradient system with the Riemann-Liouville or Caputo fractional derivatives, respectively, in the Birkhoff mechanics frame. e Hojman-Urrutia equations have an important status in the development of inverse problems in Lagrangian mechanics and Birkhoffian mechanics [36]. eir equations are written in the following form: Let a 1 � x, a 2 � y, a 3 � _ a 1 , a 4 � _ a 2 ; we can get one types of its Birkhoff expression.

Mathematical Problems in Engineering 7
Which can define the stationary state of system (60) in the sense of the Caputo fractional derivatives in equation (35).

Fractional Gradient Representation of Birkhoff System
(60) with the Riemann-Liouville Fractional Derivative. Secondly, we study fractional gradient representation of the Birkhoff system (60) using the Riemann-Liouville fractional derivative. When we take the Riemann-Liouville fractional derivative that is D a ρ � ( RL D α a ρ , ρ � 1, 2, 3, 4), (62) becomes When α � 1, we get en d 1 ω 1 ≠ 0, so (60) is not a fractional gradient system. When α � 2, d 2 ω 2 � 0, so (60) is an α order fractional gradient system with potential function with α � 2. From equation (19), we can get e potential function is 6.3. Conclusion. In this section, we give the Birkhoff expression of the Hojman-Urrutia equation and give its fractional gradient representation using the Caputo fractional derivative and the Riemann-Liouville fractional derivative, respectively. We can conclude that from this example, the Hojman-Urrutia equation in the Birkhoff mechanics frame can be expressed as an α (noninteger) order fractional gradient system using the Caputo fractional derivative or a second-order fractional gradient system using the Riemann-Liouville fractional derivative, so our results are more general than those in [31][32][33]. We also conclude that a fractional gradient system is not a classical gradient system in general. From the calculus, we find that using the Riemann-Liouville fractional derivative can only give an integer order gradient system, but using the Caputo fractional derivative can give an integer or noninteger order gradient system, so the Caputo fractional derivative is more suitable to study fractional gradient systems than the Riemann-Liouville fractional derivative.

Conclusion
In this paper, we give a definition and a general condition of a Birkhoff system to be a fractional gradient system with the Caputo fractional derivative and the Riemann-Liouville fractional derivative, respectively. When α � 1, the definition and condition degenerate into those of the classical gradient system, and when α � 2, our results goes back to those of the second order gradient representation [31][32][33], so the classical gradient system and a second-order gradient system can be considered as special cases of the fractional gradient representation of the Birkhoff system obtained in this paper.
As applications, we apply the results we get in this paper to the Hénon-Heiles equation, the Duffing oscillator model, and the Hojman-Urrutia equations. e Birkhoff equation expression and fractional gradient expression of these classical equations are given. It shows that the orders of the fractional gradient Birkhoff system are not only with α � 2 but can also be other integers or noninteger orders in this paper, so our results are more general. e limitation of this paper is that we only discuss the fractional gradient representation of an autonomous Birkhoff system. e more general cases of the fractional gradient representation of nonautonomous Birkhoff systems and the fractional gradient representation of the fractional Birkhoff system [42] will be considered in future work.

Data Availability
All data generated or analyzed during this study are included in this article. 8 Mathematical Problems in Engineering

Conflicts of Interest
e author declares that there are no conflicts of interest.