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CounterexampleConstruction: AI-adaptedVerification: AI-generatedPipeline-generatedprecheck passverified 2026-10-22 (gpt-6.1-sol)
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A derivative truncation needs its energy band

Physical scope

Pure mathematics; no physical premises or physical claim.

Statement

Assume AC. Let dp denote Lebesgue measure on the momentum line R, and let H=L2(R,dp;C) be the space of equivalence classes modulo Lebesgue-a.e. equality of complex measurable functions u satisfying ∫R∣u(p)∣2 dp<∞, with pairing ⟨u,v⟩=∫Ru(p)v(p)‾ dp, linear in its first variable, and induced norm ∥u∥2=(∫R∣u(p)∣2 dp)1/2. Define the multiplication operator A by A=Mp2,D(A)={u∈H:p2u∈H}, with (Au)(p)=p2u(p) almost everywhere. For each integer r≥1, let Ar be multiplication by p2r on D(Ar):={u∈H:p2ru∈H}; set A0=I with D(A0)=H. Fix M>0, 0≤E<M, and let N be a nonnegative integer. This is the no-band case for the low-energy remainder estimate: the polynomial approximation to (M2+p2)−1 has multiplier remainder rN(p2)=M−2(−p2/M2)N+11+p2/M2. Let rN(A) denote multiplication by rN(p2) on the maximal domain D(rN(A)):={u∈H:rN(p2)u∈H}. There is a unit vector u∈⋂r=0N+1D(Ar) for which ∥rN(A)u∥>M−2(E2/M2)N+1. Thus the low-energy factor in the remainder estimate cannot hold uniformly on the full momentum spectrum. Here p,M,E have mass units, dp has mass units, A has mass-squared units, and ℏ=c=1. This is a mathematical counterexample to removing the spectral-band hypothesis; it is not a measured physical failure.

Facts & Assumptions

Given: The complex L2 space, the multiplier operator A, M>0, 0≤E<M, and N≥0.

[A1]

The Axiom of Choice: every family of nonempty sets has a choice function.

[A2]

AC supplies countable selections and prescribed serial paths: AC implies Countable Choice, which is also required by the multiplication and L2 Hilbert suppliers.

[F1]

Low-momentum matching has a controlled remainder: its estimate assumes J belongs to the spectral subspace EA([0,E2])H.

[F2]

Spectral measure of a real L2 multiplication operator: for real measurable multiplier m, Mm on D(Mm)={u:mu∈L2} is self-adjoint and its spectral projections are multiplication by indicators of m−1(B).

[F3]

The space Lp(μ) as the quotient by null functions: L2 consists of measurable complex functions modulo equality almost everywhere.

[F4]

L2 with the integral pairing is a Hilbert space: on complex L2, ⟨f,g⟩=∫fg‾ is a Hilbert-space inner product with ∥f∥22=∫∣f∣2.

Proof

technique · direct multiplier calculation and compactly supported unit vectors

Given: The hypotheses and notation in the Statement.

1.1A1A2F2F3F4givenalgebra

By [A1]–[A2], the AC and Countable Choice assumptions of [F2] and [F4] hold. Apply [F2] with m(p)=p2 on the sigma-finite positive-measure space (R,dp); sigma-finiteness follows from R=⋃n≥1[−n,n] and [−1,1] has positive finite measure. It gives the self-adjoint nonnegative multiplication operator A with the displayed domain and spectral calculus. The multiplier (M2+p2)−1 is bounded by M−2 and maps H into D(A) because p2/(M2+p2)≤1; pointwise multiplication then shows it is the two-sided inverse of M2I+A. For u with bounded support, every Aru is in H. Pointwise finite geometric division gives

(M2+p2)−1−M−2∑r=0N(−p2/M2)r=rN(p2),

so the remainder on such u is multiplication by rN(p2).

2.1A1F1F2F3F4step 1.1algebra∎

The target coefficient CN:=M−2(E2/M2)N+1 is finite and is strictly less than M−2. If N=0, ∣r0(q)∣=M−2(q/M2)/(1+q/M2)→M−2>C0 as q→∞. If N≥1, ∣rN(q)∣=M−2(q/M2)N+1/(1+q/M2)→∞. In either case choose P>0 so that ∣rN(p2)∣>CN for every p∈[P,P+M], and put u=M−1/21[P,P+M]. The interval has Lebesgue measure M, so ∥u∥22=M−1∫PP+M1 dp=1; its bounded support puts it in every D(Ar), while ∥rN(A)u∥22=M−1∫PP+M∣rN(p2)∣2 dp>CN2. Thus the full-space bound fails. The restriction E<M is precisely what makes the proposed low-energy coefficient smaller than the N=0 high-momentum limit; the example does not contradict the band-limited theorem.

Depends on

Dependency tree · 2 levels

Sources