Alphabeta Math
TheoremStatement: Literature-sourcedProof: AI-generatedPipeline-generatedjudge pass (gpt-6-luna)
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Global error from uniform one-step defect and stability

Statement

Let d,N,p≥1, T>0, h=T/N, and tn=nh. Let y:[0,T]→Rd solve an initial-value problem in an open time-state domain D. Use the Euclidean norm ∥v∥2=(∑i=1dvi2)1/2 of The Euclidean inner product ⟨x,y⟩=∑k<nxkyk on Rn, whose triangle inequality is proved in Cauchy-Schwarz ∣⟨x,y⟩∣≤∥x∥2∥y∥2 with its equality case, the triangle inequality for ∥⋅∥2, the parallelogram law and polarisation. Let T⊂D be a common compact time-state tube with positive margin in D, and write Tt:={v∈Rd:(t,v)∈T}. Suppose a one-step map Ψh,n is defined on Ttn, produces yn+1=Ψh,n(yn), and the entire exact path, all numerical grid states, and every method stage lie in T. If constants C,C0,L≥0, independent of N and n, satisfy ∥y0−y(0)∥2≤C0hp,∥Ψh,n(y(tn))−y(tn+1)∥2≤Chp+1, and ∥Ψh,n(u)−Ψh,n(v)∥2≤(1+Lh)∥u−v∥2 for every u,v∈Ttn and 0≤n<N, then, for L>0, max⁡0≤n≤N∥yn−y(tn)∥2≤(C0exp⁡(LT)+Cexp⁡(LT)−1L)hp, and, for L=0, max⁡0≤n≤N∥yn−y(tn)∥2≤(C0+CT)hp.

Facts & Assumptions

Given: An exact first-order initial-value problem, the stated one-step map and tube, and the local-defect, stability, and initial-error bounds.

[F1]

A solution of a first-order system satisfies x(t0)=x0 and x′(t)=F(t,x(t)) on its interval (First-order systems, initial value problems, and solutions on intervals).

[F2]

For a one-step map, the local defect is the difference between its exact-start output and the exact next state; uniform local order p bounds its norm by Chp+1, and one-step Lipschitz stability bounds changes in the output by (1+Lh) times the input change (One-step ODE methods, local defect, uniform order, and step stability).

[F4]

exp⁡ is the real exponential defined by its factorial power series, with e:=exp⁡(1) (The real exponential function and the number e by a power series).

[L1]

For h=T/N and en+1≤(1+Lh)en+Chp+1 with e0≤C0hp, the finite recurrence has the two bounds stated in this item's conclusion (Finite geometric error bound from a one-step recurrence).

[A1]

The one-step map is defined on each stated time fiber, the entire exact path, numerical grid states and method stages remain in T, and its uniform local defect and fiberwise stability inequalities in the Statement hold.

Proof

1.1F1F2F3A1algebra

By [F1], each y(tn) is a defined reference state of the exact initial-value problem; [F2] identifies the two uniform inequalities in [A1] as the one-step stability and local-defect bounds. Put en:=∥yn−y(tn)∥2. The Euclidean triangle inequality [F3] and those bounds give en+1≤∥Ψh,n(yn)−Ψh,n(y(tn))∥2+∥Ψh,n(y(tn))−y(tn+1)∥2≤(1+Lh)en+Chp+1.

2.1A1F4L1step 1.1∎

Apply [L1] to the nonnegative sequence (en)n=0N using the recurrence from step 1.1 and the initial bound in [A1]. The two cases L>0 and L=0 give exactly the displayed conclusions, with exponential notation as in [F4].

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