and the integration operator J. (Usually J is used instead of I to avoid confusion with other I-like glyphs and identities.)
In this context, the term powers refers to iterative application of a linear operator acting on a function, in some analogy to function composition acting on a variable, e.g., f 2(x) = f(f(x)). For example, one may ask the question of meaningfully interpreting
as an analog of the functional square root for the differentiation operator (an operator half iterated), i.e., an expression for some linear operator that when applied twice to any function will have the same effect as differentiation.
More generally, one can look at the question of defining the linear functional
for real-number values of a in such a way that when a takes an integer value, n, the usual power of n-fold differentiation is recovered for n > 0, and the −nth power of J when n < 0.
The motivation behind this extension to the differential operator is that the semigroup of powers Da will form a continuous semigroup with parameter a, inside which the original discrete semigroup of Dn for integer n can be recovered as a subgroup. Continuous semigroups are prevalent in mathematics, and have an interesting theory. Notice here that fraction is then a misnomer for the exponent a, since it need not be rational; the use of the term fractional calculus is merely conventional.
Fractional differential equations (also known as extraordinary differential equations) are a generalization of differential equations through the application of fractional calculus.
- 1 Nature of the fractional derivative
- 2 Heuristics
- 3 Fractional derivative of a basic power function
- 4 Laplace transform
- 5 Fractional integrals
- 6 Fractional derivatives
- 7 Generalizations
- 8 Functional calculus
- 9 Applications
- 10 History
- 11 See also
- 12 Notes
- 13 Further reading
- 14 History of fractional calculus
- 15 External links
Nature of the fractional derivative
An important point is that the fractional derivative at a point x is a local property only when a is an integer; in non-integer cases we cannot say that the fractional derivative at x of a function f depends only on values of f very near x, in the way that integer-power derivatives certainly do. Therefore it is expected that the theory involves some sort of boundary conditions, involving information on the function further out. To use a metaphor, the fractional derivative requires some peripheral vision.
As far as the existence of such a theory is concerned, the foundations of the subject were laid by Liouville in a paper from 1832. The fractional derivative of a function to order a is often now defined by means of the Fourier or Mellin integral transforms.
A fairly natural question to ask is whether there exists a linear operator H, or half-derivative, such that
It turns out that there is such an operator, and indeed for any a > 0, there exists an operator P such that
or to put it another way, the definition of dny/dxn can be extended to all real values of n.
Let f(x) be a function defined for x > 0. Form the definite integral from 0 to x. Call this
Repeating this process gives
and this can be extended arbitrarily.
The Cauchy formula for repeated integration, namely
leads in a straightforward way to a generalization for real n.
Using the gamma function to remove the discrete nature of the factorial function gives us a natural candidate for fractional applications of the integral operator.
This is in fact a well-defined operator.
It is straightforward to show that the J operator satisfies
where in the last step we exchanged the order of integration and pulled out the f(s) factor from the t integration. Changing variables to r defined by t = s + (x − s)r,
The inner integral is the beta function which satisfies the following property:
Substituting back into the equation
Interchanging α and β shows that the order in which the J operator is applied is irrelevant and completes the proof.
This relationship is called the semigroup property of fractional differintegral operators. Unfortunately the comparable process for the derivative operator D is significantly more complex, but it can be shown that D is neither commutative nor additive in general.
Fractional derivative of a basic power function
Let us assume that f(x) is a monomial of the form
The first derivative is as usual
Repeating this gives the more general result that
For and , we obtain the half-derivative of the function as
Repeating this process yields
which is indeed the expected result of
For negative integer power k, the gamma function is undefined and we have to use the following relation:
This extension of the above differential operator need not be constrained only to real powers. For example, the (1 + i)th derivative of the (1 − i)th derivative yields the 2nd derivative. Also notice that setting negative values for a yields integrals.
For a general function f(x) and 0 < α < 1, the complete fractional derivative is
For arbitrary α, since the gamma function is undefined for arguments whose real part is a negative integer and whose imaginary part is zero, it is necessary to apply the fractional derivative after the integer derivative has been performed. For example,
We can also come at the question via the Laplace transform. Noting that
etc., we assert
as expected. Indeed, given the convolution rule
and shorthanding p(x) = xα−1 for clarity, we find that
which is what Cauchy gave us above.
Laplace transforms "work" on relatively few functions, but they are often useful for solving fractional differential equations.
Riemann–Liouville fractional integral
The classical form of fractional calculus is given by the Riemann–Liouville integral, which is essentially what has been described above. The theory for periodic functions (therefore including the 'boundary condition' of repeating after a period) is the Weyl integral. It is defined on Fourier series, and requires the constant Fourier coefficient to vanish (thus, it applies to functions on the unit circle whose integrals evaluate to 0).
By contrast the Grünwald–Letnikov derivative starts with the derivative instead of the integral.
Hadamard fractional integral
The Hadamard fractional integral is introduced by J. Hadamard  and is given by the following formula,
Not like classical Newtonian derivatives, a fractional derivative is defined via a fractional integral.
Riemann–Liouville fractional derivative
The corresponding derivative is calculated using Lagrange's rule for differential operators. Computing n-th order derivative over the integral of order (n − α), the α order derivative is obtained. It is important to remark that n is the nearest integer bigger than α.
Caputo fractional derivative
There is another option for computing fractional derivatives; the Caputo fractional derivative. It was introduced by M. Caputo in his 1967 paper. In contrast to the Riemann Liouville fractional derivative, when solving differential equations using Caputo's definition, it is not necessary to define the fractional order initial conditions. Caputo's definition is illustrated as follows.
The following list summaries the fractional derivatives defined in the literature.
Other fractional derivatives include:
- Grünwald–Letnikov derivative
- Hadamard derivative
- Erdélyi–Kober derivative
- Riesz derivative
- Riesz-Miller derivative
- Coimbra derivative
- Miller–Ross derivative
- Machado derivative
- Hilfer derivative
- Davidson derivative
- Chen derivative
- Chen-Machado derivative
- Udita derivative
- Weyl derivative
A recent generalization introduced by Udita Katugampola (2011) is the following, which generalizes the Riemann-Liouville fractional integral and the Hadamard fractional integral. It is given by,
Even though the integral operator in question is a close resemblance of the famous Erdélyi–Kober operator, it is not possible to obtain the Hadamard fractional integral as a direct consequence of the Erdélyi–Kober operator. Also, there is a Udita-type fractional derivative, which generalizes the Riemann-Liouville and the Hadamard fractional derivatives. As with the case of fractional integrals, the same is not true for the Erdélyi–Kober operator.
In the context of functional analysis, functions f(D) more general than powers are studied in the functional calculus of spectral theory. The theory of pseudo-differential operators also allows one to consider powers of D. The operators arising are examples of singular integral operators; and the generalisation of the classical theory to higher dimensions is called the theory of Riesz potentials. So there are a number of contemporary theories available, within which fractional calculus can be discussed. See also Erdélyi–Kober operator, important in special function theory (Kober 1940), (Erdélyi 1950–51).
Fractional conservation of mass
As described by Wheatcraft and Meerschaert (2008), a fractional conservation of mass equation is needed to model fluid flow when the control volume is not large enough compared to the scale of heterogeneity and when the flux within the control volume is non-linear. In the referenced paper, the fractional conservation of mass equation for fluid flow is:
Fractional advection dispersion equation
Time-space fractional diffusion equation models
Anomalous diffusion processes in complex media can be well characterized by using fractional-order diffusion equation models. The time derivative term is corresponding to long-time heavy tail decay and the spatial derivative for diffusion nonlocality. The time-space fractional diffusion governing equation can be written as
A simple extension of fractional derivative is the variable-order fractional derivative, the α, β are changed into α(x, t), β(x, t). Its applications in anomalous diffusion modeling can be found in reference.
Structural damping models
Acoustical wave equations for complex media
The propagation of acoustical waves in complex media, e.g. biological tissue, commonly implies attenuation obeying a frequency power-law. This kind of phenomenon may be described using a causal wave equation which incorporates fractional time derivatives:
See also  and the references therein. Such models are linked to the commonly recognized hypothesis that multiple relaxation phenomena give rise to the attenuation measured in complex media. This link is further described in  and in the survey paper, as well as the acoustic attenuation article. See  for a recent paper which compares fractional wave equations which model power-law attenuation.
Fractional Schrödinger equation in quantum theory
where the solution of the equation is the wavefunction ψ(r, t) - the quantum mechanical probability amplitude for the particle to have a given position vector r at any given time t, and ħ is the reduced Planck constant. The potential energy function V(r, t) depends on the system.
Further, Δ = ∂2/ is the Laplace operator, and Dα is a scale constant with physical dimension [Dα] = erg1 − α·cmα·sec−α, (at α = 2, D2 = 1/2m for a particle of mass m), and the operator (−ħ2Δ)α/2 is the 3-dimensional fractional quantum Riesz derivative defined by
The index α in the fractional Schrödinger equation is the Lévy index, 1 < α ≤ 2.
In applied mathematics and mathematical analysis, fractional derivative is a derivative of any arbitrary order, real or complex. Even though the term fractional is a misnomer, it has been widely accepted for such a derivative for a long time. The concept of a fractional derivative is coined by the famous mathematician Leibniz in 1695 in his letter to L'Hôpital.
- The Fractional Calculus, by Oldham, K.; and Spanier, J. Hardcover: 234 pages. Publisher: Academic Press, 1974. ISBN 0-12-525550-0
- B. Ross, "A brief history and exposition of the fundamental theory of fractional calculus", in Fractional Calculus and Its Applications. Lecture Notes in Mathematics. Vol.457. (1975) 1–36.
- J. Machado, V. Kiryakova, F. Mainardi, "Recent history of fractional calculus", Communications in Nonlinear Science and Numerical Simulation. Vol.16. No.3. (2011) 1140–1153.
- L. Debnath, "A brief historical introduction to fractional calculus", International Journal of Mathematical Education in Science and Technology. Vol.35. No.4. (2004) 487–501.
- J. Machado, A.M. Galhano, J.J. Trujillo, "On development of fractional calculus during the last fifty years", Scientometrics. Vol.98. No.1. (2014) 577–582.
- J. Machado, A.M. Galhano, J.J. Trujillo, "Science metrics on fractional calculus development since 1966", Fractional Calculus and Applied Analysis. Vol.16. No.2. (2013) 479–500.
- Fractional Integrals and Derivatives: Theory and Applications, by Samko, S.; Kilbas, A.A.; and Marichev, O. 1993, 1006 pages. ISBN 2-88124-864-0
- Acoustic attenuation
- Differential equation
- Fractional dynamics
- Fractional Fourier transform
- Fractional Schrödinger equation
- Autoregressive fractionally integrated moving average
- For the history of the subject, see the thesis (in French): Stéphane Dugowson, Les différentielles métaphysiques (histoire et philosophie de la généralisation de l'ordre de dérivation), Thèse, Université Paris Nord (1994)
- Bologna, Mauro, Short Introduction to Fractional Calculus, Universidad de Tarapaca, Arica, Chile
- Hadamard, J., Essai sur l'étude des fonctions données par leur développement de Taylor, Journal of pure and applied mathematics, vol. 4, no. 8, pp. 101–186, 1892.
- Caputo, Michele (1967). "Linear model of dissipation whose Q is almost frequency independent-II". Geophys. J. R. Astr. Soc. 13: 529–539.
- Katugampola, U.N., A New Approach To Generalized Fractional Derivatives, Bull. Math. Anal. App. Vol 6, Issue 4, 15 October 2014, pages 1–15
- Erdélyi, Arthur (1950–51). "On some functional transformations". Rendiconti del Seminario Matematico dell'Università e del Politecnico di Torino 10: 217–234. MR 0047818.
- Kober, Hermann (1940). "On fractional integrals and derivatives". The Quarterly Journal of Mathematics (Oxford Series) 11 (1): 193–211. doi:10.1093/qmath/os-11.1.193.
- Katugampola, U.N., New Approach To A Generalized Fractional Integral, Appl. Math. Comput. Vol 218, Issue 3, 1 October 2011, pages 860–865
- Wheatcraft, S., Meerschaert, M., (2008). "Fractional Conservation of Mass." Advances in Water Resources 31, 1377–1381.
- Benson, D., Wheatcraft, S., Meerschaert, M., (2000). "Application of a fractional advection-dispersion equation." Water Resources Res 36, 1403–1412.
- Benson, D., Wheatcraft, S., Meerschaert, M., (2000). "The fractional-order governing equation of Lévy motion." Water Resources Res 36, 1413–1423.
- Benson, D., Schumer, R., Wheatcraft, S., Meerschaert, M., (2001). "Fractional dispersion, Lévy motion, and the MADE tracer tests." Transport Porous Media 42, 211–240.
- Metzler, R., Klafter, J., (2000). "The random walk's guide to anomalous diffusion: a fractional dynamics approach." Phys. Rep., 339, 1-77.
- Chen, W., Sun, H.G., Zhang, X., Korosak, D., (2010). "Anomalous diffusion modeling by fractal and fractional derivatives." Computers and Mathematics with Applications, 59(5), 1754-1758. 
- Sun, H.G., Chen, W., Chen, Y.Q., (2009). "Variable-order fractional differential operators in anomalous diffusion modeling." Physica A, 2009, 388: 4586-4592.
- Nolte, Kempfle and Schäfer (2003). "Does a Real Material Behave Fractionally? Applications of Fractional Differential Operators to the Damped Structure Borne Sound in Viscoelastic Solids", Journal of Computational Acoustics (JCA), Volume 11, Issue 3.
- S. Holm and S. P. Näsholm, "A causal and fractional all-frequency wave equation for lossy media," Journal of the Acoustical Society of America, Volume 130, Issue 4, pp. 2195–2201 (October 2011)
- S. P. Näsholm and S. Holm, "Linking multiple relaxation, power-law attenuation, and fractional wave equations," Journal of the Acoustical Society of America, Volume 130, Issue 5, pp. 3038-3045 (November 2011).
- S. P. Näsholm and S. Holm, "On a Fractional Zener Elastic Wave Equation," Fract. Calc. Appl. Anal. Vol. 16, No 1 (2013), pp. 26-50, DOI: 10.2478/s13540-013--0003-1 Link to e-print
- Holm S., Näsholm, S. P., "Comparison of Fractional Wave Equations for Power Law Attenuation in Ultrasound and Elastography," Ultrasound Med. Biol., 40(4), pp. 695-703, DOI: 10.1016/j.ultrasmedbio.2013.09.033 Link to e-print
- N. Laskin, (2000), Fractional Quantum Mechanics and Lévy Path Integrals. Physics Letters 268A, 298-304.
- N. Laskin, (2002), Fractional Schrödinger equation, Physical Review E66, 056108 7 pages. (also available online: http://arxiv.org/abs/quant-ph/0206098)
- Fractional Integrals and Derivatives: Theory and Applications, by Samko, S.; Kilbas, A.A.; and Marichev, O. Hardcover: 1006 pages. Publisher: Taylor & Francis Books. ISBN 2-88124-864-0
- Theory and Applications of Fractional Differential Equations, by Kilbas, A. A.; Srivastava, H. M.; and Trujillo, J. J. Amsterdam, Netherlands, Elsevier, February 2006. ISBN 0-444-51832-0 (http://www.elsevier.com/wps/find/bookdescription.cws_home/707212/description#description)
- An Introduction to the Fractional Calculus and Fractional Differential Equations, by Kenneth S. Miller, Bertram Ross (Editor). Hardcover: 384 pages. Publisher: John Wiley & Sons; 1 edition (May 19, 1993). ISBN 0-471-58884-9
- The Fractional Calculus; Theory and Applications of Differentiation and Integration to Arbitrary Order (Mathematics in Science and Engineering, V), by Keith B. Oldham, Jerome Spanier. Hardcover. Publisher: Academic Press; (November 1974). ISBN 0-12-525550-0
- Fractional Differential Equations. An Introduction to Fractional Derivatives, Fractional Differential Equations, Some Methods of Their Solution and Some of Their Applications., (Mathematics in Science and Engineering, vol. 198), by Igor Podlubny. Hardcover. Publisher: Academic Press; (October 1998) ISBN 0-12-558840-2
- Fractional Calculus and Waves in Linear Viscoelasticity: An Introduction to Mathematical Models. by F. Mainardi, Imperial College Press, 2010. 368 pages.
- Fractional Dynamics: Applications of Fractional Calculus to Dynamics of Particles, Fields and Media. by V.E. Tarasov, Springer, 2010. 450 pages.
- Fractional Derivatives for Physicists and Engineers by V.V. Uchaikin, Springer, Higher Education Press, 2012, 385 pages.
- Fractional Calculus - An Introduction for Physicists by R. Herrmann, World Scientific, Singapore 2014. 500 pages.
- Fractals and Fractional Calculus in Continuum Mechanics, by A. Carpinteri (Editor), F. Mainardi (Editor). Paperback: 348 pages. Publisher: Springer-Verlag Telos; (January 1998). ISBN 3-211-82913-X
- Physics of Fractal Operators, by Bruce J. West, Mauro Bologna, Paolo Grigolini. Hardcover: 368 pages. Publisher: Springer Verlag; (January 14, 2003). ISBN 0-387-95554-2
- Special Functions in Fractional Calculus and Related Fractional Differintegral Equations. y Hari M Srivastava, World Scientific, Singapore, 2014. 300 pages
- Fractional Calculus: Theory and Applications. by Varsha Daftardar-gejji, Narosa Publishing House, 2013. 222 pages
- Basic Theory of Fractional Differential Equations. by Y. Zhou, World Scientific, Singapore, 201).
History of fractional calculus
- B. Ross, "A brief history and exposition of the fundamental theory of fractional calculus", in Fractional Calculus and Its Applications. Lecture Notes in Mathematics. Vol.457. (1975) 1-36.
- J. Tenreiro Machado, V. Kiryakova, F. Mainardi, "Recent history of fractional calculus", Communications in Nonlinear Science and Numerical Simulation. Vol.16. No.3. (2011) 1140–1153.
- L. Debnath, "A brief historical introduction to fractional calculus", International Journal of Mathematical Education in Science and Technology. Vol.35. No.4. (2004) 487-501.
- J.A. Tenreiro Machado, A.M.S.F. Galhano, J.J. Trujillo, "On development of fractional calculus during the last fifty years", Scientometrics. Vol.98. No.1. (2014) 577-582.
- J.A. Tenreiro Machado, A.M. Galhano, J.J. Trujillo, "Science metrics on fractional calculus development since 1966", Fractional Calculus and Applied Analysis. Vol.16. No.2. (2013) 479-500.
- Eric W. Weisstein. "Fractional Differential Equation." From MathWorld — A Wolfram Web Resource.
- MathWorld - Fractional calculus
- MathWorld - Fractional derivative
- Fractional Calculus at MathPages
- Specialized journal: Fractional Calculus and Applied Analysis
- Specialized journal: Fractional Differential Equations (FDE)
- Specialized journal: Communications in Fractional Calculus (ISSN 2218-3892)
- unr.edu (Broken Link)
- Igor Podlubny's collection of related books, articles, links, software, etc.
- GigaHedron - Richard Herrmann's collection of books, articles, preprints, etc.
- History, Definitions, and Applications for the Engineer (PDF), by Adam Loverro, University of Notre Dame
- Fractional Calculus Modelling
- Introductory Notes on Fractional Calculus
- Pseudodifferential operators and diffusive representation in modeling, control and signal
- Power Law & Fractional Dynamics
- The CRONE (R) Toolbox, a Matlab and Simulink Toolbox dedicated to fractional calculus, which is freely downloadable