The scalar-weight group W(P) of A₀ (Definition 7.1, Proposition 7.2) #
weights-and-parameters.tex. For a finite ray category Q,
W(Q) = ℤ⟨[r] : r ≠ 0, 1⟩ / ⟨[rs] − [r] − [s] : rs ≠ 0⟩ (def:scalar-weight-group).
For P = ray A₀ we use the deck-homogeneous pieces of A₀ as the nonzero rays
(Proposition 4.14: each homogeneous ray line lies in one deck degree and the
products of the normed basis are the ray products): generators [x, y, g] for
the nonzero pieces A₀(x, y)_g, relations [x, z, gh] = [x, y, g] + [y, z, h]
when a product of the two pieces is nonzero (WP).
"Every additive weight on L, with values in any abelian group, is a difference
of object values" (Proposition 7.2(2), L_addWeight_eq_diff), from the generic
cov_addWeight_eq_diff and Lemma 4.13 (ratio_hyp). For Proposition 7.2(1) see
Ψ, Φ below.
Deviation in Proposition 7.2(1): the deck group of the formalization is the
fundamental group of the coned complex (DeckGrading.lean), so the lifted
projective category need not be connected. We use the stabilizer H of the
component of a base object in place of G = π₁(K); for connected A the paper's
argument then gives W(P) ≅ ℤ^{n−1} ⊕ H_ab (Φ ∘ Ψ = 1). Its freeness, needed
later, follows: W(P) is finitely generated and embeds in a torsion-free group.
The lifted projective category L.
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Lemma 4.13, additively: an Additive kˣ-valued weight on L is a
difference of object values.
Proposition 7.2(2) (prop:integral-ray-weights) for A₀: every additive
weight on L, with values in any abelian group, is a difference of object values.
The group W(P) #
A deck piece A₀(x, y)_g.
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A nonzero piece, that is, a nonzero ray of P.
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The relations of W(P).
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The scalar-weight group W(P) (Definition 7.1).
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The class [r] of a ray.
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The tautological weight on L and its object values #
The weight of a line of L: the class of its ray.
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The deck action on L.
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An object of L.
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Connectedness in L.
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The stabilizer of the base component #
The base object (x₀, 1).
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The stabilizer H of the component of the base object.
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The connectedness of A₀: a zigzag of nonzero pieces from x₀ to every label.
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A lift of each label in the base component.
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The comparison W(P) → ℤ^{labels} × H_ab and its left inverse #
The target ℤ^{labels} × H_ab.
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The value of the comparison on a generator.
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The comparison Ψ : W(P) → ℤ^{labels} × H_ab.
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The deck classes θ_h = [e_{h x} − e_x], h ∈ H.
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The inverse comparison Φ : ℤ^{labels} × H_ab → W(P).
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Φ ∘ Ψ = 1 (Proposition 7.2(1), injectivity of the comparison).
Freeness of W(P) #
W(P) is finitely generated.
W(P) is a free abelian group of finite rank (Proposition 7.2(1)).