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ExampleConstruction: AI-generatedVerification: AI-generatedPipeline-generatedjudge pass (gpt-6-luna)verified 2026-10-22 (gpt-6.1-sol)
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Range invariance can hold when the tangential cone condition fails

Example

Let U=(−6,6)2⊂R2, let ϕ(s):=s+9∫0se−u2 du, and define the partially observed map F:U→R by F(s,t):=ϕ(s). With basepoint x0=(0,0), set Z=R2, K(a,b):=10a, and r(s,t):=(ϕ(s)/10,t). Then F(x)−F(x0)=Kr(x) for every x∈U, and r is bi-Lipschitz on U. Nevertheless, the local tangential cone condition at x0 fails on radius 5 for every allowed constant 0≤η<1. The second parameter t is unobserved: all points (0,t) give the datum 0.

Facts & Assumptions

Given: The real Hilbert spaces X=R2 and Y=R, the open set U=(−6,6)2, the basepoint x0=(0,0), and the function and operators in the Example.

[F1]

Differential range invariance for Banach-space maps: range invariance at x0 means F(x)−F(x0)=Kr(x) for a fixed bounded linear K and a map r.

[F2]

Local tangential cone condition: on a closed ball about x0, the local tangential cone condition requires ∥F(x0+h)−F(x0)−F′(x0)h∥≤η∥F(x0+h)−F(x0)∥ for every admissible h, with 0≤η<1.

Verification

1.1givenconstructalgebra

Differentiating gives ϕ′(s)=1+9e−s2, so 1≤ϕ′(s)≤10. For s>s′, directly from the integral, s−s′≤ϕ(s)−ϕ(s′)≤10(s−s′). Thus ϕ is strictly increasing and both s↦ϕ(s)/10 and its inverse are Lipschitz; with the unchanged second coordinate, r is bi-Lipschitz on U.

2.1F1givenstep 1.1algebra

The linear map K(a,b)=10a is bounded since ∣K(a,b)∣≤10a2+b2. Also F(x0)=0 and Kr(s,t)=10(ϕ(s)/10)=ϕ(s)=F(s,t), so [F1] holds throughout U. Further, F′(x0)(h1,h2)=10h1, whose kernel contains the unobserved t-axis.

2.2F2givenstep 1.1algebra

The closed radius-5 ball about x0 lies in U. At h=(5,0) the TCC remainder is ∣ϕ(5)−50∣. Since ∫05e−u2 du≤1+∫1∞ue−u2 du=1+e−1/2<3/2, we have 0<ϕ(5)<5+27/2<25. Consequently ∣ϕ(5)−50∣=50−ϕ(5)>ϕ(5)=∣F(x0+h)−F(x0)∣. This contradicts [F2] for every η<1.

3.1givenstep 2.1algebra∎

For every t∈(−6,6), F(0,t)=ϕ(0)=0. Hence the observation does not distinguish the points (0,t), even though the range-invariance factorization from step 2.1 holds.

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