In
mathematics, time-scale calculus is a unification of the theory of
difference equations with that of
differential equation
In mathematics, a differential equation is an equation that relates one or more unknown functions and their derivatives. In applications, the functions generally represent physical quantities, the derivatives represent their rates of change, an ...
s, unifying integral and differential
calculus
Calculus, originally called infinitesimal calculus or "the calculus of infinitesimals", is the mathematical study of continuous change, in the same way that geometry is the study of shape, and algebra is the study of generalizations of arithm ...
with the
calculus of finite differences
A finite difference is a mathematical expression of the form . If a finite difference is divided by , one gets a difference quotient. The approximation of derivatives by finite differences plays a central role in finite difference methods for the ...
, offering a formalism for studying
hybrid systems. It has applications in any field that requires simultaneous modelling of discrete and continuous data. It gives a new definition of a derivative such that if one differentiates a function defined on the real numbers then the definition is equivalent to standard differentiation, but if one uses a function defined on the integers then it is equivalent to the
forward difference operator.
History
Time-scale calculus was introduced in 1988 by the German mathematician
Stefan Hilger
Stefan may refer to:
* Stefan (given name)
* Stefan (surname)
* Ștefan, a Romanian given name and a surname
* Štefan, a Slavic given name and surname
* Stefan (footballer) (born 1988), Brazilian footballer
* Stefan Heym, pseudonym of German writ ...
.
However, similar ideas have been used before and go back at least to the introduction of the
Riemann–Stieltjes integral, which unifies sums and integrals.
Dynamic equations
Many results concerning differential equations carry over quite easily to corresponding results for difference equations, while other results seem to be completely different from their
continuous counterparts.
The study of dynamic equations on time scales reveals such discrepancies, and helps avoid proving results twice—once for differential equations and once again for difference equations. The general idea is to prove a result for a dynamic equation where the domain of the unknown
function
Function or functionality may refer to:
Computing
* Function key, a type of key on computer keyboards
* Function model, a structured representation of processes in a system
* Function object or functor or functionoid, a concept of object-oriente ...
is a so-called time scale (also known as a time-set), which may be an arbitrary closed subset of the reals. In this way, results apply not only to the
set
Set, The Set, SET or SETS may refer to:
Science, technology, and mathematics Mathematics
*Set (mathematics), a collection of elements
*Category of sets, the category whose objects and morphisms are sets and total functions, respectively
Electro ...
of
real numbers or set of
integers but to more general time scales such as a
Cantor set
In mathematics, the Cantor set is a set of points lying on a single line segment that has a number of unintuitive properties. It was discovered in 1874 by Henry John Stephen Smith and introduced by German mathematician Georg Cantor in 1883.
T ...
.
The three most popular examples of
calculus
Calculus, originally called infinitesimal calculus or "the calculus of infinitesimals", is the mathematical study of continuous change, in the same way that geometry is the study of shape, and algebra is the study of generalizations of arithm ...
on time scales are
differential calculus,
difference calculus, and
quantum calculus. Dynamic equations on a time scale have a potential for applications such as in
population dynamics. For example, they can model insect populations that evolve continuously while in season, die out in winter while their eggs are incubating or dormant, and then hatch in a new season, giving rise to a non-overlapping population.
Formal definitions
A time scale (or measure chain) is a
closed subset
In geometry, topology, and related branches of mathematics, a closed set is a set whose complement is an open set. In a topological space, a closed set can be defined as a set which contains all its limit points. In a complete metric space, a ...
of the
real line
In elementary mathematics, a number line is a picture of a graduated straight line that serves as visual representation of the real numbers. Every point of a number line is assumed to correspond to a real number, and every real number to a poin ...
. The common notation for a general time scale is
.
The two most commonly encountered examples of time scales are the real numbers
and the
discrete time scale
.
A single point in a time scale is defined as:
:
Operations on time scales
The ''forward jump'' and ''backward jump'' operators represent the closest point in the time scale on the right and left of a given point
, respectively. Formally:
:
(forward shift/jump operator)
:
(backward shift/jump operator)
The ''graininess''
is the distance from a point to the closest point on the right and is given by:
:
For a right-dense
,
and
.
For a left-dense
,
Classification of points
For any
,
is:
* ''left dense'' if
* ''right dense'' if
* ''left scattered'' if
* ''right scattered'' if
* ''dense'' if both left dense and right dense
* ''isolated'' if both left scattered and right scattered
As illustrated by the figure at right:
* Point
is ''dense''
* Point
is ''left dense'' and ''right scattered''
* Point
is ''isolated''
* Point
is ''left scattered'' and ''right dense''
Continuity
Continuity of a time scale is redefined as equivalent to density. A time scale is said to be ''right-continuous at point
'' if it is right dense at point
. Similarly, a time scale is said to be ''left-continuous at point
'' if it is left dense at point
.
Derivative
Take a function:
:
(where R could be any
Banach space
In mathematics, more specifically in functional analysis, a Banach space (pronounced ) is a complete normed vector space. Thus, a Banach space is a vector space with a metric that allows the computation of vector length and distance between vect ...
, but is set to the real line for simplicity).
Definition: The ''delta derivative'' (also Hilger derivative)
exists if and only if:
For every
there exists a neighborhood
of
such that:
:
for all
in
.
Take
Then
,
,
; is the derivative used in standard
calculus
Calculus, originally called infinitesimal calculus or "the calculus of infinitesimals", is the mathematical study of continuous change, in the same way that geometry is the study of shape, and algebra is the study of generalizations of arithm ...
. If
(the
integers),
,
,
is the
forward difference operator used in difference equations.
Integration
The ''delta integral'' is defined as the antiderivative with respect to the delta derivative. If
has a continuous derivative
one sets
:
Laplace transform and z-transform
A
Laplace transform can be defined for functions on time scales, which uses the same table of transforms for any arbitrary time scale. This transform can be used to solve dynamic equations on time scales. If the time scale is the non-negative integers then the transform is equal
[ to a modified ]Z-transform
In mathematics and signal processing, the Z-transform converts a discrete-time signal, which is a sequence of real or complex numbers, into a complex frequency-domain (z-domain or z-plane) representation.
It can be considered as a discrete-tim ...
:
Partial differentiation
Partial differential equation
In mathematics, a partial differential equation (PDE) is an equation which imposes relations between the various partial derivatives of a multivariable function.
The function is often thought of as an "unknown" to be solved for, similarly to ...
s and partial difference equations are unified as partial dynamic equations on time scales.
Multiple integration
Multiple integration
In mathematics (specifically multivariable calculus), a multiple integral is a definite integral of a function of several real variables, for instance, or . Integrals of a function of two variables over a region in \mathbb^2 (the real-number ...
on time scales is treated in Bohner (2005).
Stochastic dynamic equations on time scales
Stochastic differential equations and stochastic difference equations can be generalized to stochastic dynamic equations on time scales.
Measure theory on time scales
Associated with every time scale is a natural measure defined via
:
where denotes Lebesgue measure and is the backward shift operator defined on . The delta integral turns out to be the usual Lebesgue–Stieltjes integral with respect to this measure
:
and the delta derivative turns out to be the Radon–Nikodym derivative with respect to this measure
:
Distributions on time scales
The Dirac delta and Kronecker delta
In mathematics, the Kronecker delta (named after Leopold Kronecker) is a function of two variables, usually just non-negative integers. The function is 1 if the variables are equal, and 0 otherwise:
\delta_ = \begin
0 &\text i \neq j, \\
1 & ...
are unified on time scales as the ''Hilger delta'':
:
Integral equations on time scales
Integral equations and summation equation In mathematics, a summation equation or discrete integral equation is an equation in which an unknown function appears under a summation sign. The theories of summation equations and integral equations can be unified as ''integral equations on tim ...
s are unified as integral equations on time scales.
Fractional calculus on time scales
Fractional calculus on time scales is treated in Bastos, Mozyrska, and Torres.
See also
* Analysis on fractals for dynamic equations on a Cantor set
In mathematics, the Cantor set is a set of points lying on a single line segment that has a number of unintuitive properties. It was discovered in 1874 by Henry John Stephen Smith and introduced by German mathematician Georg Cantor in 1883.
T ...
.
* Multiple-scale analysis
* Method of averaging
* Krylov–Bogoliubov averaging method
References
Further reading
*
Dynamic Equations on Time Scales
Special issue of ''Journal of Computational and Applied Mathematics'' (2002)
Special Issue of ''Advances in Difference Equations'' (2006)
Special issue of ''Nonlinear Dynamics And Systems Theory'' (2009)
External links
The Baylor University Time Scales Group
Timescalewiki.org
{{DEFAULTSORT:Time Scale Calculus
Dynamical systems
Calculus
Recurrence relations