# AN EXPANSION FORMULA FOR FRACTIONAL DERIVATIVES AND ITS APPLICATION. Abstract

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1 AN EXPANSION FORMULA FOR FRACTIONAL DERIVATIVES AND ITS APPLICATION T. M. Atanackovic 1 and B. Stankovic 2 Dedicated to Professor Ivan H. Dimovski on the occasion of his 7th birthday Abstract An expansion formula for fractional derivatives given as in form of a series involving function and moments of its k-th derivative is derived. The convergence of the series is proved and an estimate of the reminder is given. The form of the fractional derivative given here is especially suitable in deriving restrictions, in a form of internal variable theory, following from the second law of thermodynamics, when applied to linear viscoelasticity of fractional derivative type. 2 Mathematics Subject Classification: 26A33 Key Words and Phrases: fractional derivatives, moments of the k-th derivative 1. Motivation In the one dimensional isothermal theory of a viscoelastic body of generalized Kelvin-Voigt type, the stress σ is given in terms of strain ε by an equation of the form σ t) = E ε t) E α ε α) t), 1)

2 366 T. M. Atanackovic, B. Stankovic where < α < 1, E and E α are constants and u α) t) = 1 d Γ 1 α) dt u τ) t τ) a dτ, < α < 1 2) is the Riemann-Liouville fractional derivative of real order α. The important problem concerning the constitutive equation of the type 1) is to find the restrictions on coefficients α, E and E α so that the Second Law of Thermodynamics, in the form of Clausius-Duhem inequality, is satisfied. There are several approaches to this problem see for example [2], [5] and [1]). One of these approaches uses the method of internal variables. In it, in order to obtain the restrictions, one has to postulate the rate equations for internal variables. The Classical Thermodynamics deals with rate equations of involving integer first) order derivatives only. This is a consequence of the assumption that the state variables such as ε) are chosen properly, so that all relevant quantities can be expressed in terms of local values of state variables. The fractional derivative is a nonlocal operator, and therefore the formalism of Thermodynamics cannot be applied directly to the rate equations involving fractional derivatives. One way to avoid such a difficulty is to use expression for fractional derivative in terms of integer order derivatives. It is well known that for analytic function f t) see [8], p.278) the fractional derivative can be expanded in a power series of involving integer order derivatives as f α) t) = n= ) α t n α n Γ n 1 α) f n) t), where ) α = 1)n 1 Γ n α) n Γ 1 α) Γ n 1). 3) The expansion 3) is not useful for our purposes for two reasons. First, it involves derivatives of the function f of all integer) orders, and second, it could be used for analytic functions only. Our intention in this note is to derive an expansion formula for the fractional derivative of a function that will involve a function, a finite number of its integer order derivatives and time moments of a single integer order derivative. This expansion formula could be used for functions that are not analytic, but only finitely many times differentiable. Also, it offers a possibility to use internal variables method to derive the restrictions on the constitutive equations involving fractional order derivatives.

3 AN EXPANSION FORMULA FOR FRACTIONAL Preliminaries The reader not familiar with spaces of generalized functions can pass over this part. Let D R) denote the space of Schwartz s distributions cf. [9]) and S R) the space of tempered distributions. Then D R) = {T D R) ; suppt [, )} and S R) = {T S R) supp T [, )}. The spaces D and S are commutative algebras with addition and convolution as operators. If f L 1 loc R), then it defines the distribution denoted by [f]. In the sequel, { δ α), α =, 1,... α = x α 1 Γ α), α R\ {, 1,...} 4) stands for the family belonging to S. If α <, α is defined by the function f x) =, x < ; f x) = x α 1 /Γ α), x >. { α ; α R} is a semi-group: α 1 α 2 = α 1α 2 is the sign of convolution). For T D and m 1 < α < m, m N, the fractional distributional derivative D α T = α T. If T is defined by a function f C[, ), T = [f], then for α = m ν, m N, < ν <1 : α T = α [f] = m ν [f] = δ m) ν [f] ) ) x = D m ν 1 [ d m Γ ν) [f] 1 x ] f τ) dτ = dx m Γ m α) x τ) mα1, and this is the definition of the α-th fractional derivative of the function f for x > cf. [4]). In some cases we can express the relation between the distributional derivative D k [f] of a distribution [f], given by the function f, and the classical derivative f k) of the function f. Such a case is the following. Suppose that f x) is a function defined for x R, f x) =, x, ). Let f x) have all derivatives in all points except at {x n } n N, { } lim x n = in which f p), p N can have jumps f n p) f, p) n < n n N, n N. Let f p) x), p k be a function defined as f p) x) = f p) x), x [, ] \ {x 1,..., x r }, {x 1,..., x r } are jump points of f k) ; f p) x n ) is not defined for n = 1,..., r, p k. Then cf.[9], p.37), [ D k [f] = f k) ] r n=1 f k 1) n δ x n ) r f n δ k 1) x n ). 5) n=1

4 368 T. M. Atanackovic, B. Stankovic We write V n V n f p) f p) ), n N, for the n-th moment of the function f p) ) t) = It is easily seen that V n f p), f p) τ) τ n dτ, n N, t. 6) ) = V n f p) ). 3. Main result Theorem 1. Let m, k N and α, ν, q be real numbers with the properties: < ν < 1, α = m ν, q = k α 1 >. Suppose that: a) the function f C k [, ) and Then for t > we have f α) t) = t q Γ k α) f ) = = f k 1) ) =. 7) V f k)) t) Γ p q) 1 Γ q) p! t p V p f k)) t) p=1 K k α, f k)) t). 8) The series in 8) converges in C, ) converges uniformly on every compact set in, )). b) If f C k [, ), but has not the property 7), then D α [f] = [K k α, f k))] t k α 1 Γ k α) f ) t α Γ 1 α) f k 1) ) [K 1 k α, f k))]. 9) c) If f has the k-th derivative in all points except in the points {x 1,..., x r } [, ) in which f p), p k 1, has bounded jumps f p) i, i = 1, r, then we have D α [f] = [K 1 k α, f k))] r i=1 r i=1 f κ 1) t x i ) k α 1 i Γ k α) t x i ) α f i Γ 1 α) [K 2 k α, f k))]. 1)

5 AN EXPANSION FORMULA FOR FRACTIONAL d) If in b) and c) we have < α < 1 and k 2, then D α [f] = t β f α) t), t, ) and Γ1 β), t x i) β Γ1 β), α β k α 1, are functions. The proof can be given without using spaces of generalized functions. e) Let k > k 1 = k w >. If f has only the k 1 -th derivative in all points t [, T ] except at the points {x 1,..., x r }, T ) in which f p), p k 1 1 has bounded jumps f p) i, i = 1,..., r, then for t [, T ) we have D α [f] = D k k 1 [K )] 2 k α, f k 1). 11) P r o o f. a) Let t [ε, T ], ε >, T <. The function f α) t) defines the distribution [ f α)] D D R). Then D α [f] = α [f] = kα k [f] = tk α 1 Γ k α) Dk [f] = Since k α 1 = q >, and f k) C[, ), f α) t) = = 1 Γ k α) t q Γ k α) tk α 1 [ Γ k α) f k)]. f k) τ) t τ) q dτ We will use the binomial formula cf. [6], p.217) 1 z) q = 1 N p=1 f k) τ) 1 τ ) q dτ, t >. 12) t Γ q 1) Γ q 1 p) p! zp R N1, Γ q 1) R N1 = N 1)!Γ q N) zn1 1 N 1) 1 τ) N 1 zτ) q N 1 dτ, 13) if 1 zτ, τ [, 1]. We change the form of the coefficients in 13) using the equality Γ q 1) Γ q 1 p) p! Γ p q) 1)p =, < q N. Γ q) p!

6 37 T. M. Atanackovic, B. Stankovic Next we prove that f α) t) can be written as: f α) t) = t q Γ k α) f k) Γ p q) 1) τ) 1 p τ Γ q) p! t p=1 ) p dτ. 14) Let M = sup f k) t), < ε t T, then by the asymptotic behavior of the function Γ z) : Γ p q) 1) p τ ) p 1 C Γ q) p! t q1, t [ε, T ], ε >, 15) p 1) and τ t. From inequality 15) it follows that the series in 14) converges uniformly in [ε, T ] for every ε > and T <. This proves a). b) The function f : f t) = f t), t [, ); f t) =, t <, is such that f p) t) exists for t, p k, and f p) t) has a jump at t =, f p) ), p k 1. Then by 5), D α [f] = tk α 1 [ Γ k α) = tk α 1 [ Γ k α) f k) f k) ] ] ) f k 1) ) δ ) f ) δ k 1) ) f k 1) t k α 1 Γ k α) f ) t α Γ k α), which proves b). c) The proof of c) is just the same as the proof of b). e) To prove 11), we consider D α [f] = α [f] = k k 1 kα k 1 [f] = δ k k1) tk α 1 Γ k α) Dk 1 [f] = D k k 1 [K )] 2 k α, f k 1).

7 AN EXPANSION FORMULA FOR FRACTIONAL Since, [K 2 k α, f k 1) )] = [K )] k α, f k 1) f k1 1) t k α 1 ) Γ k α) f ) r i=1 t k k 1 α Γ 1 k k 1 α) r i=1 f i t x i ) k k 1 α Γ 1 k k 1 α) f k 1 1) t x i ) k α 1 i, 16) Γ k α) we have only to apply D k k 1 to every addend in 16). We proved in a) that the series which defines K k α, f k 1 ) ) converges in C, ), consequently the D k k 1, applied to it, can be realized by applying D k k 1 on every term of the series cf. [9], p.76). R e m a r k. If β R, then cf. [4]) J β f t) = 1 Γ β) f τ) t τ) β 1 dτ, t >, denotes the fractional integral of order β. Since J β f t) can be written as ) τ J β β 1 f t) = Γ β) f τ) t) = β) [f] = k β k [f] [ ] t kβ 1 = D k [f], 17) Γ k β) where k N and g = [g] means that [g] is the regular distribution which corresponds to the function g. The form of J β f t) given in 17) allows that Theorem 1 is applied to the fractional integral. Also it is easily seen that K 1 k β, f k) ) t) and K 2 k β, f k) ) t), β > are functions which define distributions [K 1 k β, f k))] and [K 2 k β, f k))].

8 372 T. M. Atanackovic, B. Stankovic 4. Approximation of f α) and estimate of the reminder We give an approximation of f α) in cases when it is not a generalized function cf. a) and d)) in Theorem 1. In all these cases we can do it by taking only N terms in the series given by 8). To estimate the reminder cf. 13)), let M t denote M t = max f k) τ). τ t Then t q τ ) f k) τ) R N1 dτ Γ k α) t t q f k) Γ N 1 q) τ ) τ) dτ Γ k α) Γ 1 q) N! t Γ N 1 q) M t t q Γ k α) Γ 1 q) N! t N1 τ N1 dτ Γ N 1 q) M t t q1 Γ k α) Γ 1 q) N! N 2), t [ε, T ], ε >, T <. 18) In this way we obtain t q f α) t)= V f k)) t) Γ k α) where Q N1 t) = t q Γ k α) N Γ p q) 1 Γ q) p! t p V p f k)) t) Q N1 t), p=1 An estimate of Q N1 t) is given by 18). f k) τ) R N1 τ t 19) ) dτ. 2) 5. Applications In this section we use the expansion 8) in two particular examples. E x a m p l e 1. It is known that a viscoelastic body of generalized Kelvin-Voigt type in a linear, isothermal deformation process is described by a constitutive equation in the form of so called three parameter model see

9 AN EXPANSION FORMULA FOR FRACTIONAL [7]) given by 1). Suppose further that ε t) is continuous and has continuous first derivative, except at the point t = where the first derivative has a jump ε 1). When the expansion 8) with k = 2, p = 1, r = 1, q = 1 α is used in 8), we obtain t 1 α Γ j α 1) 1 σ t) = Eε t) E α Γ 2 α) ε1) t) Γ α 1) j! t j V j ε 2)) t) with t α t 1 α ε ) Γ 1 α) ε1) Γ 2 α), 21) V j t) = j=1 ε 2) τ) τ j dτ, j = 1,... 22) Let us identify the functions V j t) in the constitutive equation 21) as internal variables ξ j, j = 1,... In the equilibrium, internal variables are governed by thermodynamic variables while they can be considered as independent in non-equilibrium evolutions. Then 21) may be written as where ϕ t) = ϕ n t) = σ t) = E ε t) E α ϕ n t) ξ n t), 23) t1 α Γ 2 α) ; t1 α Γ 2 α) i=1 Γ n α 1) Γ α 1) n! and the internal variables evolution law is 1 ; n = 1, 2,... 24) tn ξ 1) n t) = ε 2) t) t n, n =, 1,... 25) The internal variables are not components of a vector since this is ruled out by the principle of material frame indifference, see [3]), but as a collection of scalar parameters. To derive the thermodynamical restrictions on the constitutive equation 23),25) we follow the procedure used in [1] see also [5] where other procedures are discussed). Thus, we consider a rod loaded as shown in Figure 1. We assume that the rod is deformed homogeneously so that the only independent variable is the time t.

10 374 T. M. Atanackovic, B. Stankovic Figure 1: Loading configuration The length of the rod is L in the undeformed state and l t) during the deformation. The rod is loaded by the force P and F is the cross-sectional area in the undeformed state. Thus stress, σ referred to the cross-sectional area of the unloaded rod and strain ε are given by σ t) = P F and ε t) = l 1. 26) L We describe a state of the body by two types of variables: the strain ε t) and a set of internal variables ξ n t), n = 1, 2,... The equilibrium state of the unloaded body corresponds to ε =, ξ =. 27) Thus the internal energy U, the entropy S and the free energy U T S are all functions of ε, or l) and ξ n, n = 1, 2,... The Gibbs equation for the free energy U T S reads U T S) 1) = σv ε 1) n= Θ n ξ 1) n. 28) T is the temperature, assumed to be constant. Θ n is the force associated dξ with the internal variable ξ n so that Θ n n dt is the power of the force Θ n, and V is the volume of the body which is assumed to be constant. Since σ is linear in ε and ξ n see 23)), we assume that Θ n are also linear in the same variables so that

11 AN EXPANSION FORMULA FOR FRACTIONAL Θ n = γ n ε ns ξ s, 29) where γ n t) and sn t), s, n = 1,... are specified constitutive functions. Note that with 23) the force P is given as [ ] P t) = F E ε t) E α ϕ n t) ξ n t). 3) The integrability for the free energy σ/ ξ n = Θ n / ε) requires s= i=1 E α ϕ n t) = γ n. 31) Therefore, with 31) the constitutive equations for σ and Θ n become σ t) = E ε t) E α ϕ n t) ξ n t), i=1 Θ n t) = E α ϕ n t) εt) ns t)ξ s t), n =, 1,... 32) s= We return to the Gibbs equation 28) in which we replace du dt = U 1) and dξ n dt = ξ n 1) by the equations of balance of energy and of internal variable, viz. U 1) = Q 1) σv ε 1) t), ξ n 1) = ε 2) t) t n, n =, 1,..., 33) Q 1) is the heating. Thus from 27) we obtain an equation of balance of entropy in the form S 1) Q1) T = n= Θ n T ξ1) n = n= Θ n T ε2) t) t n, 34) where we have indicated that the entropy production is non-negative. The condition 34) is equivalent to the equation 4.2) of [5]. The requirement that the equilibrium configuration ε = ξ 1 ) = ξ 2 ) =... = ) is stable leads to the condition that Gibbs free energy U T S P l tends to a minimum at ε = ξ 1 ) = ξ 2 ) =... = ). This condition is equivalent to σ ε σ σ ξ 1. ξn. Θ 1 ε Θ 1 ξ 1. Θ 1 ξ n Θ n ε Θ n ξ 1. Θ n ξ n......, positive definite, 35)

12 376 T. M. Atanackovic, B. Stankovic or E E α ϕ 1. E α ϕ n. E α ϕ 1 t) 11. 1n E α ϕ n t) n1. nn......, positive definite. 36) The condition 36) is satisfied if we take, for example, E >, E α > and mn = δ nm, where δ mn is the Kronecker symbol. We analyze next 34). By using 29) this condition leads to 1 T n= { E α ϕ n t) ε t) } ε 2) τ) τ n dτ ε 2) t) t n, t. 37) Note that see 24),25)), ϕ n t) >, n = 1, 2,... It is obvious that, the condition 37) cannot be satisfied for all t and arbitrary ε t). For example, let ε = ε sin ωt with ε and ω being constants. For sufficiently small t the condition 37) is violated. Thus, the constitutive equation 1) does not satisfy the restrictions following from the second law of Thermodynamics. This result, obtained here by the method of internal variables, is in agreement with the conclusion reached in [2] by using different approach. E x a m p l e 2. Consider the problem of expanding the α th derivative of a function that is not analytic. Thus, we consider the function {, t 1 f t) = t 1) 2. 38), t 1 Its derivative of the order α = 1/2 is {, t 1 f 1/2) t) = 1 8 π 3 t 1)3/2, t 1. 39) In this case k = 2, r = 1, q = 1 α = 1/2. From 19) we have f 1/2) t 1/2 t) = V f 2)) Γ p 1/2) 1 t) Γ 2 1/2) Γ 1/2) p! t p V p Since it follows that p=1 { f 2), t 1 t) = 2, t > 1, f k)).

13 AN EXPANSION FORMULA FOR FRACTIONAL { V p f 2)), t 1 t) = 2 tp1 1 p1, t > 1, p =, 1,. Therefore the series 8) becomes, t 1 f 1/2) t) = 4t 1/2 Γp 1/2) π t 1) 2 1 πp! t p tp1 1 p1, t 1 p=1 By calculating the first ten terms N = 1 in 19)), we obtain f 1/2) t) = 4t 1 2 t 1) 1 t t 3 1 π 4 t 24 t 2 1 t t 3 1 t t t 9 1 t t 6 1 t 5 3 t 7 1 t t 1 1 t t 8 1 t 7 t 11 1 t 1... ),. 4) t 1. 41) In Figure 2 we draw the exact expression 39) and the approximation given by 41) for t 1. Figure 2: Exact and approximate value of the derivative 6. Conclusions In this note we proved an expansion formula for fractional derivatives given by 8). It contains integer derivatives up to the finite order k and time moments of k-th derivative, given by 6). The mechanical interpretation of

14 378 T. M. Atanackovic, B. Stankovic the moments, as internal variables, leads to the possibility of applying the Clausius-Duhem inequality for materials with internal variables in order to obtain the restrictions on the coefficients in the constitutive equations. Also it allows for determining series expansion of fractional derivatives for functions that are not analytic. References [1] T. M. A t a n a c k o v i c, A modified Zener model of a viscoelastic body. Continuum Mechanics and Thermodynamics, 14 22), [2] R. L. B a g l e y and P. J. T o r v i k, On the fractional calculus model of viscoelastic behavior. Journal of Rheology, ), [3] B. C o l e m a n, and M. G u r t i n, Thermodynamics with internal state variables. J. Chem. Phys ), [4] R. G o r e n f l o and F. M a i n a r d i, Fractional calculus: Integral and differential equations of fractional order. In: Fractional Calculus in Continuum Mechanics, Springer, Wien 1997), [5] A. L i o n, On the thermodynamics of fractional damping elements. Continuum Mech. Thermodyn., ), [6] W. M a g n u s and F. O b e r h e t t i n g e r, Formel und Sëtze für die speciellen Funktionene der Mathematische Physik. Springer, Berlin 1948). [7] T. P i r t z, Analysis of four-parameter fractional derivative model of real solid materials. Journal of Sound and Vibrations, ), [8] S. G. S a m k o, A. A. K i l b a s and O. I. M a r i c h e v, Fractional Integrals and Derivatives. Gordon and Breach Science Publishers 1993). [9] L. S c h w a r t z, Théorie des distributions, I. Herman, Paris 1951). [1] V. S. V l a d i m i r o v, Generalized Functions in Mathematical Physics. Mir Publishers, Moscow 1979). 1 Department of Mechanics Received: September 22, 24 University of Novi Sad 21 Novi Sad, SERBIA and MONTENEGRO 2 Department of Mathematics, University of Novi Sad 21 Novi Sad, SERBIA and MONTENEGRO

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