The method of constructing Birkhoffian and Birkhoffian funcations of mechanical system is studied. Based on the conditions of self-adjointness and integrability conditions of linear partial differential equations, a method of undetermined tensor for constructing Birkhoffian and Birkhoffian funcations of mechanical systems is given. Two examples are given to illustrate the application of the results.

As is well known, the development of analysis mechanics from Lagrangian systems to Birkhoffian systems, achieved the self-adjointness representations of the constrained mechanical systems. Based on the Cauchy-Kovalevsky theorem of the integrability conditions for partial differential equations and the converse of the Poincar lemma, it can be proved that there exists a direct universality of Birkhoff's equations for local Newtonian system by reducing Newton's equations into a first-order form, which means that all local, analytic, regular, finite-dimensional, unconstrained or holonomic, conservative or non-conservative forms always admit, in a star-shaped neighborhood of a regular point of their variables, a representation in terms of first-order Birkhoff's equations in the coordinate and time variables of the experiment. The systems whose equations of motion are represented by the first-order Birkhoff's equations on a symplectic or a contact manifold spanned by the physical variables, are called Birkhoffian systems. The theory and method of Birkhoffian dynamics are used in hadron physics, quantum physics, relativity, rotational relativity, and fractional-order dynamics. At present, for a given dynamical system, it is important and essential to determine whether a Birkhoffian function is the first integral of the system. Although the numerical approximation is an important method of solving the differential equations, the direct theoretical analysis is more helpful for refining the general integral method, and more consistent with the usual way of solving problems of analysis mechanics. In this paper, we study how to judge whether a given Birkhoffian dynamical function to be a first integral of Birkhoff's equations, based on the point of Birkhoffian dynamical functions carrying all the informationabout motion of the system, and use the thought of deriving the first integrals of Hamiltonian systems. In Section 2, the normal first-order form and the Birkhoff's equations of the equations of motion of holonomic systems are introduced. In Section 3, we prove that the Birkhoffian function of an autonomous Birkhoffian system must be a first integral, and the Birkhoffian function of a semi-autonomous system must not be a first integral. Moreover, the energy integral, cyclic integral and Hojman integral of the non-autonomous Birkhoffian systems are given. In Section 4, two examples are given to illustrate the applications of the results. In Section 5, the full text is summarized and the results are discussed. It is necessary to point out that the judging method is effective to determine whether a given Birkhoffian functions can be identified to be a first integral of Birkhoff's equations, but other new first integral cannot be found with this method. One possible method of covering the shortage is to obtain other equivalent Birkhoffian functions in terms of isotopic transformations of Birkhoff's equations, and then use our results to seek the new first integral. In addition, we also hope to develop a more direct method of obtaining the first integrals of Birkhoff's equations in the next study.

By using the calculus of variations, the conservative mechanical systems can be formulated by Lagrange's equations or Hamilton's equations, which are the basis of establishing, simplifying and integrating the equations of motion. Thus it is important to find the solutions of inverse problems for different dynamical systems so as to construct the most of the Lagrange's equations and Hamilton's equations. However, the Lagrangian or Hamiltonian formulation for a dynamical system, limited by the conditions of self-adjointness, is not directly universal if the physical variables remain without using Darboux transformations. Fortunately, Refs. [7, 11] show that based on the Cauchy-Kovalevsky theorem of the integrability conditions for partial differential equations and the converse of the Poincar lemma, it can be proved that there exists a direct universality of Birkhoff's equation for local Newtonian system by reducing the Newton's equations to a first-order form, which means that all local, analytic, regular, finite-dimensional, unconstrained or holonomic, conservative or non-conservative forms always admit, in a star-shaped neighborhood of a regular point of their variables, a representation in terms of first-order Birkhoff's equations in the coordinate and time variables of the experiment. The systems whose equations of motion are represented by the first-order Birkhoff's equations on a symplectic or a contact manifold spanned by the physical variables are called Birkhoffian systems. At present, one of the most important tasks of Birkhoffian mechanics is to study the method of constructing the Birkhoffian and Birkhoffian functions. However, due to the complexity of Birkhoffian system, there exist only a few of results in the literature. Among them, the most famous main methods in this problem are achieved by Santilli[Santilli R M 1983 Foundations of Theoretical Mechanics II (New York: Springer-Verlag) pp12-15]. But the redundant term in Santilli's second method which is used as the classical construction method, is always neglected. As a result, the calculation procedure is tedious and complicated, and the efficiency is not high. Therefore, it is necessary to simplify the Santilli's second method. In Section 2, we will review the first-order standard form of holonomic system in the frame of Cartesian coordinates, which is the starting point of our studying the Birkhoffian systems. In Section 3, the Birkhoff's equations and the key role of Birkhoffian dynamics functions for deriving Birkhoff's equations are introduced. In Section 4, the redundant items are eliminated by using some mathematical operation skills, and then a more simplified constructing method is put forward. In Section 5, the findings in this study are summarized. Through simplifying the Santilli's second method, we realize that the determining of the Birkhoff's equations by constructing the Birkhoffian functions is equivalent to the determining of its symplectic matrix. This view provides a new perspective for solving the problem of constructing the Birkhoffian functions. Finally, the simplified method is applied to Lagrangian inverse problem, and a simplified method of solving Lagrangian function is obtained.

In this paper, we mainly study the simplification and improvement of Santilli's methods in Birkhoffian system, which is a more general type of basic dynamic system. The theories and methods of Birkhoffian dynamics have been used in hadron physics, quantum physics, rotational relativity theory, and fractional dynamics. As is well known, Lagrangian inverse problem, Hamiltonian inverse problem, and Birkhoffian inverse problem are the main objects of the dynamic inverse problems. The results given by Douglas (Douglas J 1941 Trans. Amer. Math. Soc. 50 71) and Havas[Havas P 1957 Nuovo Cimento Suppl. Ser. X5 363] show that only the self-adjoint Newtonian systems can be represented by Lagrange's equations, so the Lagrangian inverse problem is not universal for a holonomic constrained mechanical system. Furthermore, from the equivalence between Lagrange's equation and Hamilton's equation, Hamiltonian inverse problem is not universal. A natural question is then raised:whether there exists a self-adjoint dynamical model whose inverse problem is universal for holonomic constrained mechanical systems, in the field of analytical mechanics.An in-depth study of this issue in the 1980s by R. M. Santilli shows that a universal self-adjoint model exists for a holonomic constrained mechanic system that satisfies the basic conditions of locality, analyticity, and formality. The Birkhoff's equation is a natural extension of the Hamilton's equation, which shows the geometric properties of a nonconservative system as a general symplectic structure. This more general symplectic structure provides the geometry for the study of the non-conservative system preserving structure algorithms. Therefore, it is particularly important to study the problem of the Birkhoffian representation for the holonomic constrained system.For the inverse problem of Birkhoff's dynamics, studied mainly are the condition under which the mechanical systems can be represented by Birkhoff's equations and the construction method of Birkhoff's functions. However, due to the extensiveness and complexity of the holonomic nonconservative system, Birkhoff's dynamical functions do not have so simple construction method as Lagrange function and Hamilton function. The research results of this issue are very few. The existing construction methods are mainly for three constructions proposed by Santilli[Santilli R M 1983 Foundations of Theoretical Mechanics Ⅱ (New York:Springer-Verlag) pp25-28], and there are still many technical problems to be solved in the applications of these methods.In order to solve these problems, this article mainly focuses on the following content. First, according to the existence theorem of Cauchy-Kovalevskaya type equations, we prove that the autonomous system always has an autonomous Birkhoffian representation. Second, a more concise method is given to prove that Santilli's second method can be simplified. An equivalent relationship implied in Santilli's third method is found, an improved Santilli's third method is proposed, and the MATLAB programmatic calculation of the method is studied. Finally, the full text is summarized and the results are discussed.