Valeri's objection measured: the principle is right (a similarity step should be large far away and small near), and our sigma = |f'|D defeats it — at x=1e4 on a 2e4 bracket the normalised residual is 0.167, so S=0.833 'similar' and the step is small five thousand step-lengths from the root. Selecting sigma = min(|f'|D, max|f| at the ends) — no new constant — improves 403 to 390 on eight hand-picked wide problems but is WORSE over the grid: APS 3288 vs 2726, CHA 3111 vs 2683, so the mode is rejected by its pre-registered rule. Its extra solves are two fun7 problems landing inside the flat zero region, i.e. the scoring artefact and not a capability. Written into Negative results as the answer to the natural objection
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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@@ -677,6 +677,22 @@ canonical set against $2527$ --- a difference of $1922$ evaluations for one
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choice of criterion. A secant step routinely collapses the error while barely
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moving the proven bracket, so bracket progress rejects steps that were
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excellent; this is the same mechanism that makes the bracket guard cost $7260$.
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\item \textbf{Making the scale independent of the bracket does not help either.}
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The natural objection to the derived scale is that it defeats the very principle
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of a similarity step: with $\sigma=|f'|D$ the normalised residual is
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$|f|/(|f'|D)$, which on the $2\times10^4$-wide bracket of $x^3-2x-5$ at
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$x=10^4$ equals $0.167$, so $S=0.833$ --- ``similar'' --- and the step is small
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although the root is some five thousand step-lengths away. The similarity is
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reading what fraction of the \emph{bracket} the Newton step is, not how far the
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iterate is from the root. Selecting the scale between the two quantities already
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computed, $\sigma=\min(|f'|D,\max(|f(A)|,|f(B)|))$ --- no new constant, a
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\texttt{min} over what is in hand --- does give a full-bracket step far away and
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Newton's step near the root, and on eight hand-picked wide problems it improves
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$403$ evaluations to $390$. Over the whole grid it is worse on both sets:
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$3288$ on the canonical set against $2726$, and $3111$ on the wide-bracket set
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against $2683$. Its apparent extra solves ($194$ instead of $193$) are two
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\texttt{fun7} problems where it lands inside the flat zero region close enough to
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satisfy the harness --- the artefact of \S\ref{sec:fun7}, not a capability.
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\item \textbf{A logarithmic change of variable does not buy the wide brackets.}
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Working in $u=\operatorname{sign}(x-c)\log(1+|x-c|/\varepsilon_x)$ makes the
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step multiplicative, which is the right diagnosis of the flat families --- CHA
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