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arXiv:math-ph/0405012v1 4 May 2004
Lagrangians with linear velocities within Riemann-Liouville
fractional derivatives
Dumitru Baleanu
1
, Tansel Avkar
2
Department of Mathematics and Computer Science, Faculty of Arts and
Sciences, C¸ ankaya University- 06530, Ankara, Turkey
Abstract
Lagrangians linear in velocities were analyzed using the fractional
calculus and the Euler-Lagrange equations were derived. Two examples
were investigated in details, the explicit solutions of Euler-Lagrange equa-
tions were obtained and the recovery of the classical results was discussed.
PACS: 11.10.Ef. Lagrangian and Hamiltonian approa ch
1 Introduction
The mathematical idea of fractional derivatives, which goes back to the seven-
teenth century, has represented the subject of int erest for various mathematicians
[1, 2 , 3]. The fractional calculus, which means the calculus of derivatives and inte-
grals of any arbitrary real or complex order is gaining a considerable importance in
various branches of science, engineering and finance [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11].
As it was observed during the past three decades, the fractional differential
calculus describes more accurately the physical systems [4, 5, 6, 12].Many applica-
tions of fractional calculus amount to replacing the time derivative in an evolution
equation with a derivative of fractional order. This is not merely a phenomeno-
logical procedure providing an additional fit parameter. One of the pro blems
encountered in this field is what kind of fr actional derivatives will replace the
integer derivative for a given problem [1, 2, 3, 13]. Depending on the specified
physical situation different authors have applied different derivatives [6, 9].
Nonconservative Lagrangia n and Hamiltonian mechanics were investigated by
Riewe within fractional calculus [14, 15]. Besides, Lagrangian and Hamiltonian
fractional sequential mechanics, the models with symmetric fractional derivative
were studied in [16, 17] and the properties of fractional differential forms were
introduced [18, 19].
Recently, an extension of the simplest f ractional variational pro blem and the
fractional variational problem of Lagrange was obtained [20]. A natural and
interesting generalization of Agrawal’s approach [20] is to apply the fractional
1
On leave of absence from Institute of Space Sciences, P.O BOX, MG-23, R 76900 Magurele-
Bucharest, Romania, E-mails: dumitru@cankaya.edu.tr, baleanu@venus.nipne.ro
2
E-Mail: avkar@cankaya.edu.tr
1
calculus to the constrained systems [21, 22]. In the present work, we apply the
concept of f ractional calculus to the Lagrangian with linear velocities.
The plan of this paper is as follows:
In Sec. 2 the Riemann-Liouville (RL) fractional derivatives were briefly presented.
In Sec. 3 the fractional Lagra ngia ns with linear velocities were constructed and
their corresponded Euler-Lagrange were a nalyzed. Sec. 4 was devoted to our
conclusions.
2 Riemann - Liouville fractional derivatives
In this section the definitions of the right and left RL derivatives as well as t heir
basic properties are briefly presented.
RL fractional derivatives [3 , 20] are defined as follows
a
D
α
t
f(t) =
1
Γ(n − α)
d
dt
!
n
t
Z
a
(t − τ)
n−α−1
f(τ)dτ , (1)
t
D
α
b
f(t) =
1
Γ(n − α)
−
d
dt
!
n
b
Z
t
(τ − t)
n−α−1
f(τ)dτ, (2)
where the order α fulfills n − 1 ≤ α < n and Γ represents the Euler’s gamma
function. The first derivative is called the RL left fractional derivative and in (2)
the expression of the RL rig ht fractional is present ed. If α is any integer, the
relations to the usual derivative are obtained as follows
a
D
α
t
f(t) =
d
dt
!
α
,
t
D
α
b
f(t) =
−
d
dt
!
α
. (3)
Under the assumptions that f (t) is continuous and p ≥ q ≥ 0 , the most
general pro perty of RL fractional derivatives can be written as
a
D
p
t
a
D
−q
t
f(t)
=
a
D
p−q
t
f(t). (4)
For p > 0 and t > a we obtain
a
D
p
t
a
D
−p
t
f(t)
= f(t), (5)
which means that the RL fractional differentiation operator is a left inverse to
the RL fractional integration operator of the same order. The relation (5) is
called the fundamental property of the RL fractional derivative. In addition, the
fractional derivative of a constant is not zero and the RL fractional derivative of
the power function (t − a)
ν
is given by
a
D
p
t
(t − a)
ν
=
Γ(ν + 1)
Γ(−p + ν + 1)
(t − a)
ν−p
, (6)
2
where ν > −1. The normal derivatives
d
n
dt
n
and
a
D
p
t
commute only if f
(j)
(a) =
0, j = 0, 1, . . . , n − 1 is fulfilled and two RL fractional derivative op erators
a
D
p
t
and
a
D
q
t
commute only if
h
a
D
p−j
t
f(t)
i
t=a
= 0 , j = 1, · · · , m (7)
and
h
a
D
q−j
t
f(t)
i
t=a
= 0 , j = 1, · · · , n . (8)
The above properties of RL fractional derivatives lead us to the conclusion that
there are many substantial differences from the usual derivatives a nd therefor e the
solutions of the differential fractional equations contain more information than
the classical ones.
3 Fractional Euler-Lagrange equations for
Lagrangians with linear velocities
Let J[q
1
, · · · , q
n
] be a functional of the form
b
Z
a
L
t, q
1
, · · · , q
n
,
a
D
α
t
q
1
, · · · ,
a
D
α
t
q
n
,
t
D
β
b
q
1
, · · · ,
t
D
β
b
q
n
dt (9)
defined on the set of f unctions q
i
(t), i = 1, · · · , n which have continuous left
RL fractional derivative of order α and right RL fractional derivative of order
β in [a, b] and satisfy the boundary conditions q
i
(a) = q
i
a
and q
i
(b) = q
i
b
. A
necessary condition for J[q
1
, · · · , q
n
] to admit a n extremum for given functions
q
i
(t), i = 1, · · · , n is that q
i
(t) satisfy Euler-Lagrange equations [20]
∂L
∂q
j
+
t
D
α
b
∂L
∂
a
D
α
t
q
j
+
a
D
β
t
∂L
∂
t
D
β
b
q
j
= 0 , j = 1, · · · , n . (10)
In the following we consider the Lagrangian with linear velocities
L = a
j
q
i
˙q
j
− V
q
i
, (11)
where a
j
(q
i
) and V (q
i
) are functions of their arguments.
The fist step is to construct the corresponding fractional generalization of
the Lagrangian given by (11). The fractional Lag rangian is not unique, in o ther
words there are several possibilities to replace the time derivative with fractional
ones. The requirement is to obtain the same Lagrangian expression if the order
α is 1. Having in mind the above considerations, for 0 < α ≤ 1, we propose two
fractional Lagrangians. The first one is as follows
3
L
′
= a
j
q
i
a
D
α
t
q
j
− V
q
i
. (12)
From (10) and (12), t he corresponding Euler-Lagrang e equations emerge as
∂a
j
(q
i
)
∂q
k
a
D
α
t
q
j
+
t
D
α
b
a
k
q
i
−
∂V (q
i
)
∂q
k
= 0 . (13)
The second Lagrangian is given by
L
′
= −a
j
q
i
t
D
α
b
q
j
− V
q
i
. (14)
Using (1 0) and (14) the corresponding Euler-Lagrange equations become
∂a
j
(q
i
)
∂q
k
t
D
α
b
q
j
+
a
D
α
t
a
k
q
i
+
∂V (q
i
)
∂q
k
= 0 . (15)
3.1 Examples
A. To illustrate our a pproa ch, let us consider the following Lagrangian
L = ˙q
1
q
2
− ˙q
2
q
1
− (q
1
− q
2
)q
3
(16)
which is a gauge invariant [23]. In this case we proposed the corresponding
fractional Lagrangian to be as
L
′
=
a
D
α
t
q
1
q
2
−
a
D
α
t
q
2
q
1
− (q
1
− q
2
)q
3
. (17)
Using (1 3), the Euler-Lagra nge equations corresponding to (17) become
q
1
= q
2
, −
a
D
α
t
q
2
− q
3
+
t
D
α
b
q
2
= 0 ,
a
D
α
t
q
1
+ q
3
−
t
D
α
b
q
1
= 0. (18)
The solution of (18) is given as follows
q
1
= q
2
, (19)
q
3
= (−
a
D
α
t
+
t
D
α
b
)q
1
. (20)
From (19) and (20) we conclude that the classical solution is obtained if α → 1.
B. Let us consider the second Lagrangian given by
L = ˙q
1
q
2
+ ˙q
3
q
4
− V (q
2
, q
3
, q
4
), (21)
where V (q
2
, q
3
, q
4
) =
−1
2
[(q
4
)
2
− 2q
2
q
3
]. We observe that (21) is a second class
constrained system in Dirac’s classification [21].
4
We propose the fractional generalization of (21) t o be as follows
L
′
= −[(
t
D
α
b
q
1
)q
2
+ (
t
D
α
b
q
3
)q
4
+ V (q
2
, q
3
, q
4
)] . (22)
From (15) and (22) the Euler-Lagrange equations are given by
a
D
α
t
q
2
= 0 , (23)
t
D
α
b
q
1
+ q
3
= 0 , (24)
a
D
α
t
q
4
+ q
2
= 0 , (25)
t
D
α
b
q
3
− q
4
= 0 . (26)
From (23), we conclude that the solution for q
2
(t) has t he form
q
2
(t) = C
1
(t − a)
α−1
. (27)
From (25) and (27) an equation for q
4
(t) is o bta in as follows
a
D
α
t
q
4
= −C
1
(t − a)
α−1
. (28)
The solution of (28) has the form
q
4
(t) = C
2
(t − a)
α−1
−
C
1
Γ(α)
t
Z
a
(−τ + t)
α−1
(τ − a)
α−1
dτ . (29)
Using (2 6) and (29), the solution of q
3
(t) becomes
q
3
(t) = C
3
(b − t)
α−1
+
C
2
Γ(α)
b
Z
t
(−t + τ)
α−1
(−a + τ)
α−1
dτ
−
C
1
Γ(α)
2
b
Z
t
(−t + τ)
α−1
τ
Z
a
(τ − η)
α−1
(η − a)
α−1
dηdτ . (30)
Finally, the equation (24) together with (30) give the solution for q
1
(t) as
follows
q
1
(t) = C
4
(b − t)
α−1
−
C
3
Γ(α)
b
Z
t
(−t + τ)
α−1
(b − τ)
α−1
dτ
−
C
2
Γ(α)
2
b
Z
t
(−t + τ)
α−1
b
Z
τ
(−τ + η)
α−1
(−a + η)
α−1
dηdτ
+
C
1
Γ(α)
3
b
Z
t
(σ − t)
α−1
b
Z
σ
(σ − η)
α−1
η
Z
a
(τ − η)
α−1
(η − a)
α−1
dτdηdσ .(31)
5
Here C
1
, C
2
, C
3
and C
4
are constants. If α → 1, a → 0, b → 1, then the standard
solutions
q
1
(t) =
C
′
4
t
3
6
−
C
′
3
t
2
2
+C
′
2
t+C
′
1
, q
2
(t) = C
′
4
, q
3
(t) =
C
′
4
2
t
2
−C
′
3
t+C
′
2
, q
4
(t) = −C
′
4
t+C
′
3
(32)
are recovered if we redefine the constants fro m (27), (29), (30) and (31 ) as
C
′
1
= C
4
− C
3
−
C
2
2
+
C
1
3
, C
′
2
=
−C
1
2
+ C
2
+ C
3
, C
′
3
= C
2
, C
′
4
= C
1
.
4 Conclusion
The Lagrangians with linear velocities were investigated using left and right R L
fractional derivatives. The corresponding fractional Lagrangians were proposed
and the fractional Euler-Lagrange equations were obtained. Although the frac-
tional Lagrangia ns contain only left or right derivatives, both derivatives are in-
volved in Euler- Lagrange equations and both played an important role in finding
the solutions. The exact solutions of the Euler-Lagrange equations were obtained
for two examples corresponding to first and second-class constrained systems.
A “gauge invariance” was reported for the first example. The solutions of the
investigated examples depend on the limits a and b and t he limiting procedure
recovered the standard results.
5 Acknowledgments
This work is partially supported by the Scientific and Technical Research Council
of Turkey. One o f the authors (D. B.) would like to tha nk R. Hilfer, O. Agrawal
and N. Engheta for providing him impo r tant references and M. Naber and M.
Henneaux for interesting discussions.
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