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In numerical analysis, von Neumann stability analysis (also known as Fourier stability analysis) is a procedure used to check the stability of finite difference schemes as applied to linear partial differential equations.[1] The analysis is based on the Fourier decomposition of numerical error and was developed at Los Alamos National Laboratory after having been briefly described in a 1947 article by British researchers Crank and Nicolson.[2] This method is an example of explicit time integration where the function that defines governing equation is evaluated at the current time. Later, the method was given a more rigorous treatment in an article[3] co-authored by John von Neumann.

Numerical stability

The stability of numerical schemes is closely associated with numerical error. A finite difference scheme is stable if the errors made at one time step of the calculation do not cause the errors to increase as the computations are continued. A neutrally stable scheme is one in which errors remain constant as the computations are carried forward. If the errors decay and eventually damp out, the numerical scheme is said to be stable. If, on the contrary, the errors grow with time the numerical scheme is said to be unstable. The stability of numerical schemes can be investigated by performing von Neumann stability analysis. For time-dependent problems, stability guarantees that the numerical method produces a bounded solution whenever the solution of the exact differential equation is bounded. Stability, in general, can be difficult to investigate, especially when the equation under consideration is nonlinear.

In certain cases, von Neumann stability is necessary and sufficient for stability in the sense of Lax–Richtmyer (as used in the Lax equivalence theorem): The PDE and the finite difference scheme models are linear; the PDE is constant-coefficient with periodic boundary conditions and have only two independent variables; and the scheme uses no more than two time levels.[4] Von Neumann stability is necessary in a much wider variety of cases. It is often used in place of a more detailed stability analysis to provide a good guess at the restrictions (if any) on the step sizes used in the scheme because of its relative simplicity.

Illustration of the method

The von Neumann method is based on the decomposition of the errors into Fourier series. To illustrate the procedure, consider the one-dimensional heat equation

ut=α2ux2

defined on the spatial interval L, which can be discretized[5] as

(1)ujn+1=ujn+r(uj+1n2ujn+uj1n)

where

r=αΔtΔx2

and the solution ujn of the discrete equation approximates the analytical solution u(x,t) of the PDE on the grid.

Define the round-off error ϵjn as

ϵjn=Njnujn

where ujn is the solution of the discretized equation (1) that would be computed in the absence of round-off error, and Njn is the numerical solution obtained in finite precision arithmetic. Since the exact solution ujn must satisfy the discretized equation exactly, the error ϵjn must also satisfy the discretized equation.[6] Thus

(2)ϵjn+1=ϵjn+r(ϵj+1n2ϵjn+ϵj1n)

is a recurrence relation for the error. Equations (1) and (2) show that both the error and the numerical solution have the same growth or decay behavior with respect to time. For linear differential equations with periodic boundary condition, the spatial variation of error may be expanded in a finite Fourier series, in the interval L, as

(3)ϵ(x)=m=1MAmeikmx

where the wavenumber km=πmL with m=1,2,,M and M=L/Δx. The time dependence of the error is included by assuming that the amplitude of error Am is a function of time. Since the error tends to grow or decay exponentially with time, it is reasonable to assume that the amplitude varies exponentially with time; hence

(4)ϵ(x,t)=m=1Meateikmx

where a is a constant.

Since the difference equation for error is linear (the behavior of each term of the series is the same as series itself), it is enough to consider the growth of error of a typical term:

(5)ϵm(x,t)=eateikmx

The stability characteristics can be studied using just this form for the error with no loss in generality. To find out how error varies in steps of time, substitute equation (5) into equation (2), after noting that

to yield (after simplification)

(6)eaΔt=1+αΔtΔx2(eikmΔx+eikmΔx2).

Using the identities

cos(kmΔx)=eikmΔx+eikmΔx2andsin2kmΔx2=1cos(kmΔx)2

equation (6) may be written as

(7)eaΔt=14αΔtΔx2sin2(kmΔx/2)

Define the amplification factor

Gϵjn+1ϵjn

The necessary and sufficient condition for the error to remain bounded is that |G|1. However,

(8)G=ea(t+Δt)eikmxeateikmx=eaΔt

Thus, from equations (7) and (8), the condition for stability is given by

(9)|14αΔtΔx2sin2(kmΔx/2)|1

For the above condition to hold at all sin2(kmΔx/2), we have

(10)αΔtΔx212

Equation (10) gives the stability requirement for the FTCS scheme as applied to one-dimensional heat equation. It says that for a given Δx, the allowed value of Δt must be small enough to satisfy equation (10).

References

43 year old Petroleum Engineer Harry from Deep River, usually spends time with hobbies and interests like renting movies, property developers in singapore new condominium and vehicle racing. Constantly enjoys going to destinations like Camino Real de Tierra Adentro.

  1. Analysis of Numerical Methods by E. Isaacson, H. B. Keller
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    15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.

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    Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.

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    Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.

    15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.

    To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010
  5. in this case, using the FTCS discretization scheme
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