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Linear algebraic relator area implies slim Cayley triangles
Statement
Assume the Axiom of Choice. Let a finite presentation have relator lengths at most and satisfy for every null word, with . Its unit-edge metric Cayley realization has uniformly slim geodesic triangles. More precisely, writing for the supremum of minsize over triangles of perimeter at most , one has where , , , and .
Facts & Assumptions
Given: Assume AC and fix the presentation, .
The realization is geodesic; a perimeter- triangle has a marked word of length , a filling with , and vertex-side Hausdorff error at most . (Algebraic relator area controls coarse filling area).
Such a filling and error imply minsize at most . (Coarse triangle minsize is bounded by square root of area).
Under AC a geodesic space with sublinear triangle-minsize function has uniformly slim triangles. (Sublinear triangle minsize implies hyperbolicity).
AC is assumed, as required by the sublinear criterion. (The Axiom of Choice).
Proof
For any chosen triangle of perimeter , its approximating word is null. The assumed area inequality and F1 give The last inequality follows by subtracting the left inner expression from the right: the difference is . All constants are nonnegative.
Apply F2 with . The triangle minsize is at most For this is at most the same expression with . Taking the supremum over those triangles proves the asserted bound for , including .
Set . This function is nonnegative and nondecreasing. For , Given , taking larger than , , and makes the right side at most . Thus , and step 2.1 gives .
F1 establishes that is geodesic, and step 3.1 verifies precisely the sublinearity hypothesis of F3. Under the assumed AC, F3 therefore yields a finite slimness constant for all chosen geodesic triangles of . The displayed depend only on ; a common slimness constant across presentations requires the separate uniformity argument.
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Sources
- Drutu–Kapovich, Geometric Group Theory — §9.7.4, Theorem 9.100 and Proposition 9.103; §11.20, Proposition 11.167(a) (standard reference, not scraped)