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fix: three false statements (Egorov domain, 10.4.1/9.3.21 Nat div, Lusin caution)#601

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Chessing234:fix/statement-bugs-egorov-exponents-lusin
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fix: three false statements (Egorov domain, 10.4.1/9.3.21 Nat div, Lusin caution)#601
Chessing234 wants to merge 7 commits into
teorth:mainfrom
Chessing234:fix/statement-bugs-egorov-exponents-lusin

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Summary

  • Egorov finite-measure refinement: converge on S ∩ Eᶜ, not Sᶜ ∪ E.
  • Exercises 10.4.1 and Example 9.3.21: use 1/(n:ℝ) so the exponent/argument is real division, not Nat.div.
  • Lusin caution: claim failure of ambient ContinuousAt on Eᶜ (ContinuousOn holds for 1_ℚ).

Feature branches merged here: fix/egorov-finite-measure-domain, fix/nat-div-real-exponents, fix/lusin-caution-continuous-at.

Test plan

  • lake build Analysis.MeasureTheory.Section_1_3_5 Analysis.Section_10_4 Analysis.Section_9_3
  • CI Build book

Chessing234 and others added 7 commits July 23, 2026 12:47
With n : ℕ, (1/n:ℝ) is Nat.div (0 for n ≥ 2), so x^(1/n) collapses
to the constant 1 and the stated derivative is false.

Co-authored-by: Cursor <cursoragent@cursor.com>
The finite-measure refinement should converge on S outside the small
exceptional set, not on the complement Sᶜ ∪ E.

Co-authored-by: Cursor <cursoragent@cursor.com>
(1/n:ℝ) for n : ℕ is Nat.div, so the sequence is eventually f(0)
instead of f(1/n) along positive rationals.

Co-authored-by: Cursor <cursoragent@cursor.com>
ContinuousOn (1_ℚ) (ℝ \ ℚ) holds, so the old claim is false; the
intended caution is that ambient ContinuousAt can fail on Eᶜ.

Co-authored-by: Cursor <cursoragent@cursor.com>
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