Finite difference method
In numerical analysis, finitedifference methods (FDM) are discretizations used for solving differential equations by approximating them with difference equations that finite differences approximate the derivatives.
Differential equations  

Navier–Stokes differential equations used to simulate airflow around an obstruction.  
Classification  
Types


Relation to processes 

Solution  
General topics 

Solution methods 

FDMs convert a linear ordinary differential equations (ODE) or nonlinear partial differential equations (PDE) into a system of equations that can be solved by matrix algebra techniques. The reduction of the differential equation to a system of algebraic equations makes the problem of finding the solution to a given ODE/PDE ideally suited to modern computers, hence the widespread use of FDMs in modern numerical analysis[1]. Today, FDMs are the dominant approach to numerical solutions of PDEs.[1]
Derivation from Taylor's polynomial
First, assuming the function whose derivatives are to be approximated is properlybehaved, by Taylor's theorem, we can create a Taylor series expansion
where n! denotes the factorial of n, and R_{n}(x) is a remainder term, denoting the difference between the Taylor polynomial of degree n and the original function. We will derive an approximation for the first derivative of the function "f" by first truncating the Taylor polynomial:
Setting, x_{0}=a we have,
Dividing across by h gives:
Solving for f'(a):
Assuming that is sufficiently small, the approximation of the first derivative of "f" is:
Accuracy and order
The error in a method's solution is defined as the difference between the approximation and the exact analytical solution. The two sources of error in finite difference methods are roundoff error, the loss of precision due to computer rounding of decimal quantities, and truncation error or discretization error, the difference between the exact solution of the original differential equation and the exact quantity assuming perfect arithmetic (that is, assuming no roundoff).
To use a finite difference method to approximate the solution to a problem, one must first discretize the problem's domain. This is usually done by dividing the domain into a uniform grid (see image to the right). This means that finitedifference methods produce sets of discrete numerical approximations to the derivative, often in a "timestepping" manner.
An expression of general interest is the local truncation error of a method. Typically expressed using BigO notation, local truncation error refers to the error from a single application of a method. That is, it is the quantity if refers to the exact value and to the numerical approximation. The remainder term of a Taylor polynomial is convenient for analyzing the local truncation error. Using the Lagrange form of the remainder from the Taylor polynomial for , which is
, where ,
the dominant term of the local truncation error can be discovered. For example, again using the forwarddifference formula for the first derivative, knowing that ,
and with some algebraic manipulation, this leads to
and further noting that the quantity on the left is the approximation from the finite difference method and that the quantity on the right is the exact quantity of interest plus a remainder, clearly that remainder is the local truncation error. A final expression of this example and its order is:
This means that, in this case, the local truncation error is proportional to the step sizes. The quality and duration of simulated FDM solution depends on the discretization equation selection and the step sizes (time and space steps). The data quality and simulation duration increase significantly with smaller step size.[2] Therefore, a reasonable balance between data quality and simulation duration is necessary for practical usage. Large time steps are useful for increasing simulation speed in practice. However, time steps which are too large may create instabilities and affect the data quality.[3][4]
The von Neumann and CourantFriedrichsLewy criteria are often evaluated to determine the numerical model stability.[3][4][5][6]
Example: ordinary differential equation
For example, consider the ordinary differential equation
The Euler method for solving this equation uses the finite difference quotient
to approximate the differential equation by first substituting it for u'(x) then applying a little algebra (multiplying both sides by h, and then adding u(x) to both sides) to get
The last equation is a finitedifference equation, and solving this equation gives an approximate solution to the differential equation.
Example: The heat equation
Consider the normalized heat equation in one dimension, with homogeneous Dirichlet boundary conditions
 (boundary condition)
 (initial condition)
One way to numerically solve this equation is to approximate all the derivatives by finite differences. We partition the domain in space using a mesh and in time using a mesh . We assume a uniform partition both in space and in time, so the difference between two consecutive space points will be h and between two consecutive time points will be k. The points
will represent the numerical approximation of
Explicit method
Using a forward difference at time and a secondorder central difference for the space derivative at position (FTCS) we get the recurrence equation:
This is an explicit method for solving the onedimensional heat equation.
We can obtain from the other values this way:
where
So, with this recurrence relation, and knowing the values at time n, one can obtain the corresponding values at time n+1. and must be replaced by the boundary conditions, in this example they are both 0.
This explicit method is known to be numerically stable and convergent whenever .[7] The numerical errors are proportional to the time step and the square of the space step:
Implicit method
If we use the backward difference at time and a secondorder central difference for the space derivative at position (The Backward Time, Centered Space Method "BTCS") we get the recurrence equation:
This is an implicit method for solving the onedimensional heat equation.
We can obtain from solving a system of linear equations:
The scheme is always numerically stable and convergent but usually more numerically intensive than the explicit method as it requires solving a system of numerical equations on each time step. The errors are linear over the time step and quadratic over the space step:
Crank–Nicolson method
Finally if we use the central difference at time and a secondorder central difference for the space derivative at position ("CTCS") we get the recurrence equation:
This formula is known as the Crank–Nicolson method.
We can obtain from solving a system of linear equations:
The scheme is always numerically stable and convergent but usually more numerically intensive as it requires solving a system of numerical equations on each time step. The errors are quadratic over both the time step and the space step:
Usually the Crank–Nicolson scheme is the most accurate scheme for small time steps. The explicit scheme is the least accurate and can be unstable, but is also the easiest to implement and the least numerically intensive. The implicit scheme works the best for large time steps.
Comparison
The figures below present the solutions given by the above methods to approximate the heat equation
with the boundary condition
The exact solution is
Example: The Laplace operator
The (continuous) Laplace operator in dimensions is given by . The discrete Laplace operator depends on the dimension .
In 1D the Laplace operator is approximated as
This approximation is usually expressed via the following stencil
and which represents a symmetric, tridiagonal matrix. For an equidistant grid one gets a Toeplitz matrix.
The 2D case shows all the characteristics of the more general nD case. Each second partial derivative needs to be approximated similar to the 1D case
which is usually given by the following stencil
Consistency
Consistency of the abovementioned approximation can be shown for highly regular functions, such as . The statement is
To proof this one needs to substitute Taylor Series expansions up to order 3 into the discrete Laplace operator.
Properties
Subharmonic
Similar to continuous subharmonic functions one can define subharmonic functions for finitedifference approximations
Mean value
One can define a general stencil of positive type via
If is (discrete) subharmonic then the following mean value property holds
where the approximation is evaluated on points of the grid, and the stencil is assumed to be of positive type.
A similar mean value property also holds for the continuous case.
Maximum principle
For a (discrete) subharmonic function the following holds
where are discretizations of the continuous domain , respectively the boundary .
A similar maximum principle also holds for the continuous case.
The SBPSAT method
The SBPSAT method is a stable and accurate technique for discretizing and imposing boundary conditions of a wellposed partial differential equation using high order finite differences.[8][9]The method is based on finite differences where the differentiation operators exhibit summationbyparts properties. Typically, these operators consist of differentiation matrices with central difference stencils in the interior with carefully chosen onesided boundary stencils designed to mimic integrationbyparts in the discrete setting. Using the SAT technique, the boundary conditions of the PDE are imposed weakly, where the boundary values are "pulled" towards the desired conditions rather than exactly fulfilled. If the tuning parameters (inherent to the SAT technique) are chosen properly, the resulting system of ODE's will exhibit similar energy behavior as the continuous PDE, i.e. the system has no nonphysical energy growth. This guarantees stability if an integration scheme with a stability region that includes parts of the imaginary axis, such as the fourth order RungeKutta method, is used. This makes the SAT technique an attractive method of imposing boundary conditions for higher order finite difference methods, in contrast to for example the injection method, which typically will not be stable if high order differentiation operators are used.
See also
 Finite element method
 Finite difference
 Finite difference time domain
 Infinite difference method
 Stencil (numerical analysis)
 Finite difference coefficients
 Fivepoint stencil
 Lax–Richtmyer theorem
 Finite difference methods for option pricing
 Upwind differencing scheme for convection
 Central differencing scheme
 Discrete Poisson equation
 Discrete Laplace operator
References
 Christian Grossmann; HansG. Roos; Martin Stynes (2007). Numerical Treatment of Partial Differential Equations. Springer Science & Business Media. p. 23. ISBN 9783540715849.
 Arieh Iserles (2008). A first course in the numerical analysis of differential equations. Cambridge University Press. p. 23. ISBN 9780521734905.
 Hoffman JD; Frankel S (2001). Numerical methods for engineers and scientists. CRC Press, Boca Raton.
 Jaluria Y; Atluri S (1994). "Computational heat transfer". Computational Mechanics. 14: 385–386. doi:10.1007/BF00377593.
 Majumdar P (2005). Computational methods for heat and mass transfer (1st ed.). Taylor and Francis, New York.
 Smith GD (1985). Numerical solution of partial differential equations: finite difference methods (3rd ed.). Oxford University Press.
 Crank, J. The Mathematics of Diffusion. 2nd Edition, Oxford, 1975, p. 143.
 Bo Strand (1994). Summation by Parts for Finite Difference Approximations for d/dx. Journal of Computational Physics. doi:10.1006/jcph.1994.1005.
 Mark H. Carpenter; David I. Gottlieb; Saul S. Abarbanel (1994). Timestable boundary conditions for finitedifference schemes solving hyperbolic systems: Methodology and application to highorder compact schemes. Journal of Computational Physics. doi:10.1006/jcph.1994.1057.
 K.W. Morton and D.F. Mayers, Numerical Solution of Partial Differential Equations, An Introduction. Cambridge University Press, 2005.
 Autar Kaw and E. Eric Kalu, Numerical Methods with Applications, (2008) . Contains a brief, engineeringoriented introduction to FDM (for ODEs) in Chapter 08.07.
 John Strikwerda (2004). Finite Difference Schemes and Partial Differential Equations (2nd ed.). SIAM. ISBN 9780898716399.
 Smith, G. D. (1985), Numerical Solution of Partial Differential Equations: Finite Difference Methods, 3rd ed., Oxford University Press
 Peter Olver (2013). Introduction to Partial Differential Equations. Springer. Chapter 5: Finite differences. ISBN 9783319020990..
 Randall J. LeVeque, Finite Difference Methods for Ordinary and Partial Differential Equations, SIAM, 2007.