Alphabeta Math
CounterexampleConstruction: AI-adaptedVerification: AI-adaptedPipeline-generatedprecheck passjudge pass (gpt-5.6-terra)audited 2026-08-29
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The first chip identifies the source of the statement or construction; the second identifies the source of its local proof or verification.

  • Literature-sourced: the exact statement appears in a cited source; only wording and notation differ.
  • AI-adapted: a semantically identical restatement of literature-sourced material, modulo indexing, notation, and boundary cases adopted by the library.
  • AI-generated: a genuinely novel statement formulated by AI, with no source for the claim itself.

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The same matrix can have two different numerical ranks under two declared tolerances

Statement refuted

Numerical rank is a property of the matrix alone, an exact invariant that does not depend on the declared tolerance.

Facts & Assumptions

Given: The diagonal matrix A=diag(1,1/2,0) with singular values σ1=1, σ2=1/2, σ3=0, and the two tolerances τ1=3/4 and τ2=1/4.

[L1]

In the spectral norm, rankτ(A)=#{j:σj>τ} (In the spectral norm, numerical rank counts the singular values above the declared threshold).

Counterexample

technique · direct
1.1

At the tolerance τ1=3/4, the only singular value strictly above the threshold is σ1=1, so [L1] gives rankτ1(A)=#{j:σj>3/4}=1.

L1algebra
1.2

At the tolerance τ2=1/4, the singular values strictly above the threshold are σ1=1 and σ2=1/2, so [L1] gives rankτ2(A)=#{j:σj>1/4}=2.

L1algebra
1.3

The singular value σ2=1/2 lies strictly between the two tolerances, τ2=1/4<σ2<τ1=3/4, which is exactly why it is counted at τ2 but not at τ1.

givenalgebra
2.1

Steps 1.1, 1.2 and 1.3 exhibit one matrix and two declared tolerances with numerical ranks 1 and 2: numerical rank is a function of the matrix together with the tolerance, not of the matrix alone, which refutes the false statement.

step 1.1step 1.2step 1.3

Depends on

Used by

Nothing in the library uses this result yet.

Dependency tree · two levels

5 results within two dependency steps of this one, each drawn at its shortest distance from it. An arrow runs from a result to what uses it, so the chart reads left to right and ends at this result, which carries a heavier outline. Every node is a link to that result. Click elsewhere on the chart to enlarge it.

Sources