fix: JordanMeasurable.boolean_algebra.not_isSigmaAlgebra needs d ≥ 1 - #646
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fix: JordanMeasurable.boolean_algebra.not_isSigmaAlgebra needs d ≥ 1#646Chessing234 wants to merge 1 commit into
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EuclideanSpace' 0 is a one-point space, so every subset of it (there are only two) is Jordan measurable and the Jordan algebra is all of the power set, which is trivially closed under countable unions. The negative statement therefore only holds in positive dimension. The neighbouring Exercise 1.4.5 statements in Section_1_4_1 and JordanMeasurable.not_borel below already carry the same d >= 1 hypothesis. Signed-off-by: Taksh <takshkothari09@gmail.com>
This was referenced Aug 15, 2026
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EuclideanSpace' 0is a one-point space, so its only subsets are∅anduniv, both Jordan measurable. The Jordan algebra in dimension 0 is therefore the full power set, which is closed under countable unions — soisSigmaAlgebraholds atd = 0and the statement as formalized is false there.Adding
(hd : d ≥ 1)matches the convention already used by the neighbouring degenerate-dimension statements: the four Exercise 1.4.5_not_atomicresults inSection_1_4_1.leanandJordanMeasurable.not_borelfurther down this same file all carryd ≥ 1.The body stays
sorry, and no other declaration references this theorem, so nothing downstream changes.