
An example curve of e^{t} cos(10t) that is added together with similar curves to form a Laplace Transform.

Animation showing how adding together curves can approximate a function.
In mathematics, the Laplace transform, named after its discoverer PierreSimon Laplace (/ləˈplɑːs/), is an integral transform that converts a function of a real variable (usually , in the time domain) to a function of a complex variable (in the complex frequency domain, also known as sdomain, or splane). The transform has many applications in science and engineering, mostly as a tool for solving linear differential equations.^{[1]} In particular, it transforms ordinary differential equations into algebraic equations and convolution into multiplication.^{[2]}^{[3]} For suitable functions f, the Laplace transform is defined by the integral
The Laplace transform is named after mathematician and astronomer PierreSimon, marquis de Laplace, who used a similar transform in his work on probability theory.^{[4]} Laplace wrote extensively about the use of generating functions (1814), and the integral form of the Laplace transform evolved naturally as a result.^{[5]}
Laplace's use of generating functions was similar to what is now known as the ztransform, and he gave little attention to the continuous variable case which was discussed by Niels Henrik Abel.^{[6]} The theory was further developed in the 19th and early 20th centuries by Mathias Lerch,^{[7]} Oliver Heaviside,^{[8]} and Thomas Bromwich.^{[9]}
The current widespread use of the transform (mainly in engineering) came about during and soon after World War II,^{[10]} replacing the earlier Heaviside operational calculus. The advantages of the Laplace transform had been emphasized by Gustav Doetsch,^{[11]} to whom the name Laplace transform is apparently due.
From 1744, Leonhard Euler investigated integrals of the form
These types of integrals seem first to have attracted Laplace's attention in 1782, where he was following in the spirit of Euler in using the integrals themselves as solutions of equations.^{[15]} However, in 1785, Laplace took the critical step forward when, rather than simply looking for a solution in the form of an integral, he started to apply the transforms in the sense that was later to become popular. He used an integral of the form
Laplace also recognised that Joseph Fourier's method of Fourier series for solving the diffusion equation could only apply to a limited region of space, because those solutions were periodic. In 1809, Laplace applied his transform to find solutions that diffused indefinitely in space.^{[17]}
The Laplace transform of a function f(t), defined for all real numbers t ≥ 0, is the function F(s), which is a unilateral transform defined by

(Eq.1) 
where s is a complex frequency domain parameter
An alternate notation for the Laplace transform is instead of F.^{[3]}
The meaning of the integral depends on types of functions of interest. A necessary condition for existence of the integral is that f must be locally integrable on [0, ∞). For locally integrable functions that decay at infinity or are of exponential type ( ), the integral can be understood to be a (proper) Lebesgue integral. However, for many applications it is necessary to regard it as a conditionally convergent improper integral at ∞. Still more generally, the integral can be understood in a weak sense, and this is dealt with below.
One can define the Laplace transform of a finite Borel measure μ by the Lebesgue integral^{[18]}
An important special case is where μ is a probability measure, for example, the Dirac delta function. In operational calculus, the Laplace transform of a measure is often treated as though the measure came from a probability density function f. In that case, to avoid potential confusion, one often writes
This limit emphasizes that any point mass located at 0 is entirely captured by the Laplace transform. Although with the Lebesgue integral, it is not necessary to take such a limit, it does appear more naturally in connection with the Laplace–Stieltjes transform.
When one says "the Laplace transform" without qualification, the unilateral or onesided transform is usually intended. The Laplace transform can be alternatively defined as the bilateral Laplace transform, or twosided Laplace transform, by extending the limits of integration to be the entire real axis. If that is done, the common unilateral transform simply becomes a special case of the bilateral transform, where the definition of the function being transformed is multiplied by the Heaviside step function.
The bilateral Laplace transform F(s) is defined as follows:

(Eq.2) 
An alternate notation for the bilateral Laplace transform is , instead of F.
Two integrable functions have the same Laplace transform only if they differ on a set of Lebesgue measure zero. This means that, on the range of the transform, there is an inverse transform. In fact, besides integrable functions, the Laplace transform is a onetoone mapping from one function space into another in many other function spaces as well, although there is usually no easy characterization of the range.
Typical function spaces in which this is true include the spaces of bounded continuous functions, the space L^{∞}(0, ∞), or more generally tempered distributions on (0, ∞). The Laplace transform is also defined and injective for suitable spaces of tempered distributions.
In these cases, the image of the Laplace transform lives in a space of analytic functions in the region of convergence. The inverse Laplace transform is given by the following complex integral, which is known by various names (the Bromwich integral, the Fourier–Mellin integral, and Mellin's inverse formula):

(Eq.3) 
where γ is a real number so that the contour path of integration is in the region of convergence of F(s). In most applications, the contour can be closed, allowing the use of the residue theorem. An alternative formula for the inverse Laplace transform is given by Post's inversion formula. The limit here is interpreted in the weak* topology.
In practice, it is typically more convenient to decompose a Laplace transform into known transforms of functions obtained from a table, and construct the inverse by inspection.
In pure and applied probability, the Laplace transform is defined as an expected value. If X is a random variable with probability density function f, then the Laplace transform of f is given by the expectation
By convention, this is referred to as the Laplace transform of the random variable X itself. Here, replacing s by −t gives the moment generating function of X. The Laplace transform has applications throughout probability theory, including first passage times of stochastic processes such as Markov chains, and renewal theory.
Of particular use is the ability to recover the cumulative distribution function of a continuous random variable X, by means of the Laplace transform as follows:^{[19]}
The Laplace transform can be alternatively defined in a purely algebraic manner by applying a field of fractions construction to the convolution ring of functions on the positive halfline. The resulting space of abstract operators is exactly equivalent to Laplace space, but in this construction the forward and reverse transforms never need to be explicitly defined (avoiding the related difficulties with proving convergence).^{[20]}
If f is a locally integrable function (or more generally a Borel measure locally of bounded variation), then the Laplace transform F(s) of f converges provided that the limit
The Laplace transform converges absolutely if the integral
The set of values for which F(s) converges absolutely is either of the form Re(s) > a or Re(s) ≥ a, where a is an extended real constant with −∞ ≤ a ≤ ∞ (a consequence of the dominated convergence theorem). The constant a is known as the abscissa of absolute convergence, and depends on the growth behavior of f(t).^{[21]} Analogously, the twosided transform converges absolutely in a strip of the form a < Re(s) < b, and possibly including the lines Re(s) = a or Re(s) = b.^{[22]} The subset of values of s for which the Laplace transform converges absolutely is called the region of absolute convergence, or the domain of absolute convergence. In the twosided case, it is sometimes called the strip of absolute convergence. The Laplace transform is analytic in the region of absolute convergence: this is a consequence of Fubini's theorem and Morera's theorem.
Similarly, the set of values for which F(s) converges (conditionally or absolutely) is known as the region of conditional convergence, or simply the region of convergence (ROC). If the Laplace transform converges (conditionally) at s = s_{0}, then it automatically converges for all s with Re(s) > Re(s_{0}). Therefore, the region of convergence is a halfplane of the form Re(s) > a, possibly including some points of the boundary line Re(s) = a.
In the region of convergence Re(s) > Re(s_{0}), the Laplace transform of f can be expressed by integrating by parts as the integral
That is, F(s) can effectively be expressed, in the region of convergence, as the absolutely convergent Laplace transform of some other function. In particular, it is analytic.
There are several Paley–Wiener theorems concerning the relationship between the decay properties of f, and the properties of the Laplace transform within the region of convergence.
In engineering applications, a function corresponding to a linear timeinvariant (LTI) system is stable if every bounded input produces a bounded output. This is equivalent to the absolute convergence of the Laplace transform of the impulse response function in the region Re(s) ≥ 0. As a result, LTI systems are stable, provided that the poles of the Laplace transform of the impulse response function have negative real part.
This ROC is used in knowing about the causality and stability of a system.
The Laplace transform has a number of properties that make it useful for analyzing linear dynamical systems. The most significant advantage is that differentiation becomes multiplication, and integration becomes division, by s (reminiscent of the way logarithms change multiplication to addition of logarithms).
Because of this property, the Laplace variable s is also known as operator variable in the L domain: either derivative operator or (for s^{−1}) integration operator. The transform turns integral equations and differential equations to polynomial equations, which are much easier to solve. Once solved, use of the inverse Laplace transform reverts to the original domain.
Given the functions f(t) and g(t), and their respective Laplace transforms F(s) and G(s),
the following table is a list of properties of unilateral Laplace transform:^{[23]}
Property  Time domain  s domain  Comment 

Linearity  Can be proved using basic rules of integration.  
Frequencydomain derivative  F′ is the first derivative of F with respect to s.  
Frequencydomain general derivative  More general form, nth derivative of F(s).  
Derivative  f is assumed to be a differentiable function, and its derivative is assumed to be of exponential type. This can then be obtained by integration by parts  
Second derivative  f is assumed twice differentiable and the second derivative to be of exponential type. Follows by applying the Differentiation property to f′(t).  
General derivative  f is assumed to be ntimes differentiable, with nth derivative of exponential type. Follows by mathematical induction.  
Frequencydomain integration  This is deduced using the nature of frequency differentiation and conditional convergence.  
Timedomain integration  u(t) is the Heaviside step function and (u ∗ f)(t) is the convolution of u(t) and f(t).  
Frequency shifting  
Time shifting  a > 0, u(t) is the Heaviside step function  
Time scaling  a > 0  
Multiplication  The integration is done along the vertical line Re(σ) = c that lies entirely within the region of convergence of F.^{[24]}  
Convolution  
Circular convolution  For periodic functions with period T.  
Complex conjugation  
Crosscorrelation  
Periodic function  f(t) is a periodic function of period T so that f(t) = f(t + T), for all t ≥ 0. This is the result of the time shifting property and the geometric series.  
Periodic summation 


The Laplace transform can be viewed as a continuous analogue of a power series.^{[25]} If a(n) is a discrete function of a positive integer n, then the power series associated to a(n) is the series
Changing the base of the power from x to e gives
For this to converge for, say, all bounded functions f, it is necessary to require that ln x < 0. Making the substitution −s = ln x gives just the Laplace transform:
In other words, the Laplace transform is a continuous analog of a power series, in which the discrete parameter n is replaced by the continuous parameter t, and x is replaced by e^{−s}.
The quantities
are the moments of the function f. If the first n moments of f converge absolutely, then by repeated differentiation under the integral,
It is often convenient to use the differentiation property of the Laplace transform to find the transform of a function's derivative. This can be derived from the basic expression for a Laplace transform as follows:
The general result
A useful property of the Laplace transform is the following:
By plugging in the lefthand side turns into:
This method can be used to compute integrals that would otherwise be difficult to compute using elementary methods of real calculus. For example,
The (unilateral) Laplace–Stieltjes transform of a function g : ℝ → ℝ is defined by the Lebesgue–Stieltjes integral
The function g is assumed to be of bounded variation. If g is the antiderivative of f:
then the Laplace–Stieltjes transform of g and the Laplace transform of f coincide. In general, the Laplace–Stieltjes transform is the Laplace transform of the Stieltjes measure associated to g. So in practice, the only distinction between the two transforms is that the Laplace transform is thought of as operating on the density function of the measure, whereas the Laplace–Stieltjes transform is thought of as operating on its cumulative distribution function.^{[26]}
The Fourier transform is a special case (under certain conditions) of the bilateral Laplace transform. While the Fourier transform of a function is a complex function of a real variable (frequency), the Laplace transform of a function is a complex function of a complex variable. The Laplace transform is usually restricted to transformation of functions of t with t ≥ 0. A consequence of this restriction is that the Laplace transform of a function is a holomorphic function of the variable s. Unlike the Fourier transform, the Laplace transform of a distribution is generally a wellbehaved function. Techniques of complex variables can also be used to directly study Laplace transforms. As a holomorphic function, the Laplace transform has a power series representation. This power series expresses a function as a linear superposition of moments of the function. This perspective has applications in probability theory.
The Fourier transform is equivalent to evaluating the bilateral Laplace transform with imaginary argument s = iω or s = 2πiξ^{[27]} when the condition explained below is fulfilled,
This convention of the Fourier transform ( in Fourier transform § Other conventions) requires a factor of 1/2π on the inverse Fourier transform. This relationship between the Laplace and Fourier transforms is often used to determine the frequency spectrum of a signal or dynamical system.
The above relation is valid as stated if and only if the region of convergence (ROC) of F(s) contains the imaginary axis, σ = 0.
For example, the function f(t) = cos(ω_{0}t) has a Laplace transform F(s) = s/(s^{2} + ω_{0}^{2}) whose ROC is Re(s) > 0. As s = iω_{0} is a pole of F(s), substituting s = iω in F(s) does not yield the Fourier transform of f(t)u(t), which is proportional to the Dirac delta function δ(ω − ω_{0}).
However, a relation of the form
The Mellin transform and its inverse are related to the twosided Laplace transform by a simple change of variables.
If in the Mellin transform
The unilateral or onesided Ztransform is simply the Laplace transform of an ideally sampled signal with the substitution of
Let
The Laplace transform of the sampled signal x_{q}(t) is
This is the precise definition of the unilateral Ztransform of the discrete function x[n]
Comparing the last two equations, we find the relationship between the unilateral Ztransform and the Laplace transform of the sampled signal,
The similarity between the Z and Laplace transforms is expanded upon in the theory of time scale calculus.
The integral form of the Borel transform
Since an ordinary Laplace transform can be written as a special case of a twosided transform, and since the twosided transform can be written as the sum of two onesided transforms, the theory of the Laplace, Fourier, Mellin, and Ztransforms are at bottom the same subject. However, a different point of view and different characteristic problems are associated with each of these four major integral transforms.
The following table provides Laplace transforms for many common functions of a single variable.^{[28]}^{[29]} For definitions and explanations, see the Explanatory Notes at the end of the table.
Because the Laplace transform is a linear operator,
Using this linearity, and various trigonometric, hyperbolic, and complex number (etc.) properties and/or identities, some Laplace transforms can be obtained from others more quickly than by using the definition directly.
The unilateral Laplace transform takes as input a function whose time domain is the nonnegative reals, which is why all of the time domain functions in the table below are multiples of the Heaviside step function, u(t).
The entries of the table that involve a time delay τ are required to be causal (meaning that τ > 0). A causal system is a system where the impulse response h(t) is zero for all time t prior to t = 0. In general, the region of convergence for causal systems is not the same as that of anticausal systems.
Function  Time domain 
Laplace sdomain 
Region of convergence  Reference  

unit impulse  all s  inspection  
delayed impulse  time shift of unit impulse  
unit step  integrate unit impulse  
delayed unit step  time shift of unit step  
rectangular impulse  
ramp  integrate unit impulse twice  
nth power (for integer n) 
(n > −1) 
integrate unit step n times  
qth power (for complex q) 

^{[30]}^{[31]}  
nth root  Set q = 1/n above.  
nth power with frequency shift  Integrate unit step, apply frequency shift  
delayed nth power with frequency shift 
integrate unit step, apply frequency shift, apply time shift  
exponential decay  Frequency shift of unit step  
twosided exponential decay (only for bilateral transform) 
Frequency shift of unit step  
exponential approach  unit step minus exponential decay  
sine  ^{[32]}  
cosine  ^{[32]}  
hyperbolic sine  ^{[33]}  
hyperbolic cosine  ^{[33]}  
exponentially decaying sine wave 
^{[32]}  
exponentially decaying cosine wave 
^{[32]}  
natural logarithm  ^{[33]}  
Bessel function of the first kind, of order n 
(n > −1) 
^{[34]}  
Error function  ^{[34]}  
Explanatory notes:

The Laplace transform is often used in circuit analysis, and simple conversions to the sdomain of circuit elements can be made. Circuit elements can be transformed into impedances, very similar to phasor impedances.
Here is a summary of equivalents:
Note that the resistor is exactly the same in the time domain and the sdomain. The sources are put in if there are initial conditions on the circuit elements. For example, if a capacitor has an initial voltage across it, or if the inductor has an initial current through it, the sources inserted in the sdomain account for that.
The equivalents for current and voltage sources are simply derived from the transformations in the table above.
The Laplace transform is used frequently in engineering and physics; the output of a linear timeinvariant system can be calculated by convolving its unit impulse response with the input signal. Performing this calculation in Laplace space turns the convolution into a multiplication; the latter being easier to solve because of its algebraic form. For more information, see control theory. The Laplace transform is invertible on a large class of functions. Given a simple mathematical or functional description of an input or output to a system, the Laplace transform provides an alternative functional description that often simplifies the process of analyzing the behavior of the system, or in synthesizing a new system based on a set of specifications.^{[35]}
The Laplace transform can also be used to solve differential equations and is used extensively in mechanical engineering and electrical engineering. The Laplace transform reduces a linear differential equation to an algebraic equation, which can then be solved by the formal rules of algebra. The original differential equation can then be solved by applying the inverse Laplace transform. English electrical engineer Oliver Heaviside first proposed a similar scheme, although without using the Laplace transform; and the resulting operational calculus is credited as the Heaviside calculus.
Let . Then (see the table above)
In the limit , one gets
The validity of this identity can be proved by other means. It is an example of a Frullani integral.
Another example is Dirichlet integral.
In the theory of electrical circuits, the current flow in a capacitor is proportional to the capacitance and rate of change in the electrical potential (with equations as for the SI unit system). Symbolically, this is expressed by the differential equation
Taking the Laplace transform of this equation, we obtain
Solving for V(s) we have
The definition of the complex impedance Z (in ohms) is the ratio of the complex voltage V divided by the complex current I while holding the initial state V_{0} at zero:
Using this definition and the previous equation, we find:
Consider a linear timeinvariant system with transfer function
The impulse response is simply the inverse Laplace transform of this transfer function:
To evaluate this inverse transform, we begin by expanding H(s) using the method of partial fraction expansion,
The unknown constants P and R are the residues located at the corresponding poles of the transfer function. Each residue represents the relative contribution of that singularity to the transfer function's overall shape.
By the residue theorem, the inverse Laplace transform depends only upon the poles and their residues. To find the residue P, we multiply both sides of the equation by s + α to get
Then by letting s = −α, the contribution from R vanishes and all that is left is
Similarly, the residue R is given by
Note that
Finally, using the linearity property and the known transform for exponential decay (see Item #3 in the Table of Laplace Transforms, above), we can take the inverse Laplace transform of H(s) to obtain
The same result can be achieved using the convolution property as if the system is a series of filters with transfer functions 1/(s + α) and 1/(s + β). That is, the inverse of
Time function  Laplace transform 

Starting with the Laplace transform,
We are now able to take the inverse Laplace transform of our terms:
This is just the sine of the sum of the arguments, yielding:
We can apply similar logic to find that
In statistical mechanics, the Laplace transform of the density of states defines the partition function.^{[36]} That is, the canonical partition function is given by
The wide and general applicability of the Laplace transform and its inverse is illustrated by an application in astronomy which provides some information on the spatial distribution of matter of an astronomical source of radiofrequency thermal radiation too distant to resolve as more than a point, given its flux density spectrum, rather than relating the time domain with the spectrum (frequency domain).
Assuming certain properties of the object, e.g. spherical shape and constant temperature, calculations based on carrying out an inverse Laplace transformation on the spectrum of the object can produce the only possible model of the distribution of matter in it (density as a function of distance from the center) consistent with the spectrum.^{[37]} When independent information on the structure of an object is available, the inverse Laplace transform method has been found to be in good agreement.
An example curve of e^{t} cos(10t) that is added together with similar curves to form a Laplace Transform.
Animation showing how adding together curves can approximate a function.
Laplace Transform and the ztransform are closely related to the Fourier Transform. Laplace Transform is somewhat more general in scope than the Fourier Transform, and is widely used by engineers for describing continuous circuits and systems, including automatic control systems.
{{citation}}
: CS1 maint: location missing publisher (link)