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Relative Salvetti models and the finiteness qualification
PDF p. 13: Remark 3.27
Statement
Let be a Coxeter system with finite and . For the relative complement of Relative complexified hyperplane complement, assume its cone interior is nonempty. The locally finite relative arrangement of Local finiteness of the interior relative arrangement has a Salvetti CW model homotopy equivalent to , by Independent Salvetti homotopy-model input. This model is finite whenever the relative arrangement is finite; in particular it is finite when is finite. Local finiteness alone does not imply finite CW homotopy type: for and , there is no finite CW model, as established in The dihedral product complement admits no finite CW model.
Proof
[F1] Given: The product-complement definition of Relative complexified hyperplane complement, the local finiteness and convexity of the relative cone arrangement from Local finiteness of the interior relative arrangement, and the independent theorem Independent Salvetti homotopy-model input. Goal: Retain the homotopy-model conclusion, state its sound finite-arrangement special case, and resolve the unrestricted finiteness assertion.
[F2] The finite-CW obstruction for the infinite-dihedral complement is supplied by The dihedral product complement admits no finite CW model.
[F3] For the infinite dihedral system, The infinite dihedral Tits cone is an open half-plane with its vertex supplies coordinates , , , , the Tits-cone interior and all root hyperplanes , . Isolated complex meridian disks inside the product of the dihedral cone defines and its meridians . Independent meridian classes detected by individual winding maps gives their independent classes in , the integral singular homology group of one-cycles modulo boundaries.
Proof technique: direct.
Apply Independent Salvetti homotopy-model input with in the finite-dimensional relative space. The assumed nonempty interior is open and convex; hyperplanes missing it are discarded, as in the arrangement convention. Local finiteness is supplied by Local finiteness of the interior relative arrangement. Its product complement is exactly Relative complexified hyperplane complement. Thus the theorem gives the claimed Salvetti homotopy model, finite when the arrangement is finite.
If is finite, the relative-complement supplier establishes and finiteness of the relative arrangement (there are only finitely many roots). Consequently the finite-arrangement clause applies.
Take , and . In Relative complexified hyperplane complement, the empty vanishing condition gives , so is the full Tits cone and the relative hyperplanes are exactly the root hyperplanes. By [F3], its interior is and these hyperplanes are precisely the , all meeting . Under , the product identifies with , and each identifies with because its real defining linear form vanishes on exactly when it vanishes on both and . Thus is exactly the space of Isolated complex meridian disks inside the product of the dihedral cone. Its meridian classes are independent in by Independent meridian classes detected by individual winding maps, and it has no finite CW model by [F2]. This finite-rank example satisfies the general Section 3 hypotheses and disproves unrestricted finiteness while leaving the homotopy-model assertion intact.
Source notes
Remark 3.27 of the original article explicitly says “has a finite CW-model”; the independent Paris theorem does not assert finiteness for infinite arrangements. The preceding sentence on spherical type is a conditional identification, not a standing assumption for the remark: §2.2 explicitly imposes spherical type only in Sections 5 and 6. Therefore the counterexample addresses the original article, rather than only a generated reconstruction. The correction preserves the cited Salvetti method and its finite spherical case; it does not claim finite type is necessary for every relative arrangement, or address equivariance under arbitrary normalizer actions. Exact PDF anchors and the explicit counterexample are retained in run-local evidence for the final assessor, who must independently compare the finding against the original PDF.
Depends on
- Relative complexified hyperplane complement
- Local finiteness of the interior relative arrangement
- Independent Salvetti homotopy-model input
- The dihedral product complement admits no finite CW model
- The infinite dihedral Tits cone is an open half-plane with its vertex
- Isolated complex meridian disks inside the product of the dihedral cone
- Independent meridian classes detected by individual winding maps
Dependency tree · 2 levels
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Sources
- Normalisers of parabolic subgroups of Artin–Tits groups and Tits cone intersections · PDF p.13, equation (3.26) and Remark 3.27; p.4, §2.2