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About the Noether’s theorem for fractional Lagrangian systems and a generalization of the classical Jost method of proof

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Veröffentlicht/Copyright: 23. Oktober 2019
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Abstract

Recently, the fractional Noether’s theorem derived by G. Frederico and D.F.M. Torres in [10] was proved to be wrong by R.A.C. Ferreira and A.B. Malinowska in (see [7]) using a counterexample and doubts are stated about the validity of other Noether’s type Theorem, in particular ([9], Theorem 32). However, the counterexample does not explain why and where the proof given in [10] does not work. In this paper, we make a detailed analysis of the proof proposed by G. Frederico and D.F.M. Torres in [9] which is based on a fractional generalization of a method proposed by J. Jost and X.Li-Jost in the classical case. This method is also used in [10]. We first detail this method and then its fractional version. Several points leading to difficulties are put in evidence, in particular the definition of variational symmetries and some properties of local group of transformations in the fractional case. These difficulties arise in several generalization of the Jost’s method, in particular in the discrete setting. We then derive a fractional Noether’s Theorem following this strategy, correcting the initial statement of Frederico and Torres in [9] and obtaining an alternative proof of the main result of Atanackovic and al. [3].

Appendix A. Note on numerical solving of the Euler-Lagrange equation

In order to obtain approximate solution for the Euler-Lagrange equation (4.39) we convert this equation into the integral form. First let us formulate useful composition rules between fractional operators according to Definitions 2.1 and 2.2 (see [5]):

Lemma A.1

Letα ∈ (0, 1) andxAC([a, b], ℝn), then the following relations

  • Ia+αcDa+αx=xx(a),

  • Ia+αDa+αx=x

are satisfied almost everywhere.

In the case of the right operators the counterparts of this rules are also valid. According to the definitions of fractional integrals (2.3) and (2.4) we can conclude that for every constant C ∈ ℝ we have

Ia+αC=(ta)αΓ(1+α)C,IbαC=(bt)αΓ(1+α)C.(1.8)

Now, the integral form of the Euler-Lagrange equation (4.34):

x(t)+Ia+αIbαx(t)tabαIa+αIbαx(t)t=b=1tabαx(a)+tabαx(b)(1.9)

can be easily derived based on (1.8), the relations between derivatives (2.7) and the composition rules defined in Lemma A.1. Note that, if we put α = 1 in (1.9), we obtain the integral form of the equation = x.

For the purpose of discretization of the integral equation (1.9) we define the equidistant partition on [a, b] : h = (ba)/N, tk = a + kh, for k = 0, …, N, N ∈ ℕ. On the subinterval [ti, ti+1] we substitute the function f by the arithmetic average of values f(ti) and f(ti+1). We derive the approximations of the integrals:

Ia+αf(t)|t=tk=1Γ(α)i=0k1titi+1f(s)(tks)1αds1Γ(α)i=0k1titi+11(tks)1αf(ti)+f(ti+1)2ds=:hIa+αf(tk)(1.10)

for k = 1, …, N, and

Ibαf(t)|t=tk=1Γ(α)i=kN1titi+1f(s)(stk)1αds1Γ(α)i=kN1titi+11(stk)1αf(ti)+f(ti+1)2ds=:hIbαf(tk)(1.11)

for k = 0, …, N – 1, where the sub-integrals can be directly calculated. Then we obtain the following algebraic system of equations

X0=x(a),Xk+hIa+αhIbαXktkabαhIa+αhIbαXN=1tkabαX0+tkabαXN,XN=x(b)(1.12)

which gives an approximate solution of the Euler-Lagrange equation (4.39).

References

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Received: 2018-08-12
Published Online: 2019-10-23
Published in Print: 2019-08-27

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