† Corresponding author. E-mail:
Supported by the National Natural Science Foundation of China under Grant Nos. 11272227 and 11572212, the Innovation Program for Postgraduate in Higher Education Institutions of Jiangsu Province (KYZZ16 0479), and the Innovation Program for Postgraduate of Suzhou University of Science and Technology (SKCX16 058)
The aim of this paper is to study the Herglotz variational principle of the fractional Birkhoffian system and its Noether symmetry and conserved quantities. First, the fractional Pfaff-Herglotz action and the fractional Pfaff-Herglotz principle are presented. Second, based on different definitions of fractional derivatives, four kinds of fractional Birkhoff’s equations in terms of the Herglotz variational principle are established. Further, the definition and criterion of Noether symmetry of the fractional Birkhoffian system in terms of the Herglotz variational problem are given. According to the relationship between the symmetry and the conserved quantities, the Noether’s theorems within four different fractional derivatives are derived, which can reduce to the Noether’s theorem of the Birkhoffian system in terms of the Herglotz variational principle under the classical conditions. As applications of the Noether’s t heorems of the fractional Birkhoffian system in terms of the Herglotz variational principle, an example is given at the end of this paper.
As is well known, the symmetry and the conserved quantity play important roles in the fields of mathematics, physics, dynamics, optimal control, and so on. The symmetry of a mechanical system is described by the invariance under an infinitesimal transformation, which has a profound influence on the dynamic behaviors and qualitative properties of a system.[1] The conserved quantity can reduce the dimensions and simplify the integral of the differential equation via reducing the degrees of freedom of a system. In 1918, Emmy Noether[2] noted the relationship between the symmetry and the conservation quantity and put forward Noether’s theorem. Since Noether’s theorem explains all the conservation laws of Newtonian mechanics, the studies of Noether’s symmetry and the conserved quantity have been one of the hot topics in the study of analytical mechanics and their applications in recent decades. So far, Noether symmetry and the conserved quantity have been studied in Lagrangian systems,[3–5] Hamiltonian systems,[6–8] Birkhoffian systems[9–11] as well as nonholonomic systems,[12–13] and so on. Not only that, but some scholars have studied Noether symmetry and conserved quantity in the model of fractional calculus.
The origin of the concept of fractional calculus was advanced in 1695 when L'Hopital and Leibniz discussed the significance of a function in the order of 1/2. However, the theories of fractional calculus were rarely studied because of the research difficulties and ambiguity of the research significance. Until in the end of the 1970s, Mandelbrot[14] discovered that a large number of fractional dimension examples exists in nature. Then, it is found that fractional calculus has a wide range of applications in quantum mechanics, chaotic dynamics, long-range dissipation, signal processing and so on.[15–19] In recent years, various models of fractional integral and derivative have been developed, such as Riemann-Liouville fractional derivatives, Caputo fractional derivatives, Riesz-Riemann-Liouville fractional derivatives, Riesz-Caputo fractional derivatives, and so on. In this paper, we will study these four kinds of fractional derivatives. In addition, fractional calculus has applied in a variety of mechanical systems.[20–29] Since Birkhoffian systems are natural generalizations of Lagrangian systems and Hamiltonian systems, it is significant to propose the theory of fractional Birkhoffian systems. Up to now, there are a series of results and applications of fractional Birkhoffian systems.[30–36] Besides, in 2014, Almeida and Malinowska[37] considered the fractional Herglotz variational principle, where fractionality stands in the dependence of the Lagrangian by Caputo fractional derivatives of Herglotz variables.
Herglotz variational principle,[38] proposed by Gustav Herglotz in 1930 firstly, gives a variational principle description of nonconservative systems even when the Lagrangian does not depend on time. The functional of Herglotz variational principle is defined by a differential equation, which generalizes the classical ones defining the functional by an integral. Before Georgieva and Guenther,[39] Noether’s theorems were applicable only to the classical variational principle and were not applied to the functional defined by different equations. Torres and his co-workers presented Noether’s theorem of higher-order variational problems of Herglotz type[40] and Noether’s first theorem based on Herglotz variational problems with time delay.[41] Besides, they also proposed Noether’s theorem for fractional Herglotz variational problems.[42–43] Zhang studied Noether’s theorem based on Herglotz variational problems in phase space and of Birkhoffian system.[44–46] However, applications of fractional Birkhoffian systems for the Herglotz variational principle have been not investigated in previous works.
In this paper, we will study Noether symmetry and conserved quantities of the fractional Birkhoffian system in terms of the Herglotz variational problem. First of all, a brief summery of fractional derivatives and their properties are presented in Sec.
For the convenience of readers, we introduce the representations of Riemann-Liouville derivatives, Caputo derivatives, Riesz-Riemann-Liouville derivatives and Riesz-Caputo derivatives. Assume that the function f(ξ) is continuous and integrable in every finite interval (a,t) and (t,b). The left and the right Riemann-Liouville derivatives are[47]
In this paper, we will need formulae for fractional integration by parts as follows given in Ref. [49]:
According to the ideas of the generalized variational principle proposed by Herglotz,[38] the Herglotz variational problem of the fractional Birkhoffian system can be formulated as follows.
Determine the trajectories aν(t) satisfying the boundary conditions
Taking the calculation of the variation to Eq. (
Using the above Pfaff-Herglotz principle, we can deduce to fractional Birkhoff’s equations in terms of the Herglotz variational problem based on different definitions of fractional derivatives.
Let
Introducing the infinitesimal transformations of r-parameter finite transformation group Gr with respect to time t and Birkhoff’s variables aν, they are
For any function F(t), we have ΔF = δF + ḞΔt. And noting the commutative relation (d/dt)δF = δ(d/dt)F = δḞ, we can get easily
According to the concepts of Noether symmetry,[2] we can establish the definition and criterion of Noether symmetric transformations of the fractional Birkhoffian system in terms of the Herglotz variational problem as follows.
Try to find the conserved quantities of the following fractional Birkhoffian system in terms of the Herglotz variational problem
First, substituting Eqs. (
Next, substituting Eqs. (
Similarly, when γ = 1/2 and β = α, we can obtain the Noether’s theorem of the fractional Birkhoffian system in terms of the Herglotz variational problem based on Riesz-Caputo derivatives
In this paper, we define the fractional Pfaff-Herglotz action and present the fractional Pfaff-Herglotz principle firstly. And then, fractional Birkhoff’s equations, criterion of Noether symmetry and Noether’s theorems of the fractional Birkhoffian system in terms of the Herglotz variational problem based on four different definitions of fractional derivatives are obtained. The theorems not only can reduce to the Noether’s theorem of the Birkhoffian system for the Herglotz variational problem under classical conditions, but also can become the Noether’s theorem of the Birkhoffian system when the functional z is independent of time. The traditional Lagrangian, Hamiltonian and Birkhoffian systems, the fractional Lagrangian, Hamiltonian and Birkhoffian systems, as well as the traditional Lagrangian, Hamiltonian and Birkhoffian systems for the Herglotz variational problem are special cases of the fractional Birkhoffian system for the Herglotz variational problem. Obviously, the method and results in this letter are of more universal significance. Besides, fractional Herglotz variational principle provides an effective method to deal with fractional conservative and nonconservative systems systematically. Therefore, fractional mechanical systems in terms of the Herglotz variational problem may be taken a deeper study in future.
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