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Partial converse to Proposition 3(1): goodness and the identity section

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QAlgorithms.Papers.Independent.Intakethird100.P73.good_group_wellSectioned_of_degree_zero

by Gohan · Nov 1, 2026 · Mathlib 0df444a (Lean v4.33.1)

prove2quantum

Let BBB be a weakly Hausdorff k-space and let GGG be a simplicial group object in K/B\mathcal K/BK/B. Thus its levels GnG_nGn​ are parametrized groups and every simplicial structure map is a continuous homomorphism over BBB. Suppose GGG is good: every degeneracy si:Gn→Gn+1s_i:G_n\to G_{n+1}si​:Gn​→Gn+1​, for n≥0n\ge0n≥0 and 0≤i≤n0\le i\le n0≤i≤n, is a strong fˉ\bar ffˉ​-cofibration. If G0G_0G0​ is well-sectioned, then

∀n≥0,1Gn:B⟶Gn is a strong fˉ-cofibration.\forall n\ge0,\qquad 1_{G_n}:B\longrightarrow G_n\text{ is a strong }\bar f\text{-cofibration}.∀n≥0,1Gn​​:B⟶Gn​ is a strong fˉ​-cofibration.

Here 1Gn1_{G_n}1Gn​​ is the canonical identity section, and well-sectionedness means exactly that this section has the strong cofibration property. Strong cofibrations are defined by their left lifting property against all fiberwise acyclic fiberwise covering-homotopy fibrations in K/B\mathcal K/BK/B. No fibrancy hypothesis is imposed on GGG.

This gives the source's partial converse: goodness propagates well-sectionedness from degree zero to every degree.

Preamble
import Definitions.Def_QAlgorithms_Papers_Independent_Intakethird100_P73
Formal statement
noncomputable section

universe u

namespace QAlgorithms.Papers.Independent.Intakethird100.P73

open CategoryTheory

open scoped Topology

/-- Roberts--Stevenson, the partial converse following Proposition 3,
arXiv:1203.2460v4, PDF p. 13. A good simplicial parametrized group whose
zero-simplices are well-sectioned is well-sectioned in every degree.
No fibrancy is assumed. -/
theorem good_group_wellSectioned_of_degree_zero
    (B : KSpace) (hB : IsWeaklyHausdorff B) (G : SimplicialParametrizedGroup B)
    (hgood : IsGoodSimplicialGroup G)
    (hzero : IsWellSectioned (G.obj (Opposite.op (SimplexCategory.mk 0)))) :
    ∀ n : ℕ, IsWellSectioned (G.obj (Opposite.op (SimplexCategory.mk n))) := by sorry

end QAlgorithms.Papers.Independent.Intakethird100.P73

end
Source
Simplicial principal bundles in parametrized spaces, arXiv:1203.2460v4, PDF p. 13, Partial converse to Proposition 3(1)
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What the Lean code literally says, in plain math · gpt-6.1-sol

Let BBB be a compactly generated space such that the image of every continuous map S→BS\to BS→B from any compact Hausdorff space SSS in the same universe is closed in BBB. Let GGG be a simplicial parametrized group over BBB: it assigns a parametrized group to every object of the opposite simplex category and continuous maps over BBB preserving units and multiplication to its morphisms, with pointwise identity and composition laws. Denote its degree-nnn total space by EnE_nEn​, its projection by πn:En→B\pi_n:E_n\to Bπn​:En​→B, and its unit section by en:B→Ene_n:B\to E_nen​:B→En​; each EnE_nEn​ is compactly generated, inversion is continuous over BBB, and multiplication is continuous on the compactly generated replacement of the pairs in the same fiber and satisfies the group laws there. To state the hypotheses and conclusion explicitly, write I=[0,1]I=[0,1]I=[0,1], and let k(T)k(T)k(T) retain the set of TTT with the finest topology making all originally continuous maps from compact Hausdorff spaces in the same universe continuous. Maps over BBB are continuous maps commuting with projections; a homotopy over BBB from f0:S→Tf_0:S\to Tf0​:S→T to f1:S→Tf_1:S\to Tf1​:S→T is a continuous F:k(S×I)→TF:k(S\times I)\to TF:k(S×I)→T with F(s,0)=f0(s)F(s,0)=f_0(s)F(s,0)=f0​(s), F(s,1)=f1(s)F(s,1)=f_1(s)F(s,1)=f1​(s), and πT(F(s,t))=πS(s)\pi_T(F(s,t))=\pi_S(s)πT​(F(s,t))=πS​(s). For a map r:P→Qr:P\to Qr:P→Q over BBB, let C(r)\mathcal C(r)C(r) mean the following lifting property: for all compactly generated spaces U,VU,VU,V over BBB and maps p:U→Vp:U\to Vp:U→V over BBB satisfying both conditions below, all maps a:P→Ua:P\to Ua:P→U and b:Q→Vb:Q\to Vb:Q→V over BBB with p∘a=b∘rp\circ a=b\circ rp∘a=b∘r admit a map ℓ:Q→U\ell:Q\to Uℓ:Q→U over BBB with ℓ∘r=a\ell\circ r=aℓ∘r=a and p∘ℓ=bp\circ\ell=bp∘ℓ=b. The first condition on ppp is that, for every compactly generated space ZZZ over BBB, map c:Z→Uc:Z\to Uc:Z→U over BBB, and continuous H:k(Z×I)→VH:k(Z\times I)\to VH:k(Z×I)→V with πV(H(z,t))=πZ(z)\pi_V(H(z,t))=\pi_Z(z)πV​(H(z,t))=πZ​(z) and H(z,0)=p(c(z))H(z,0)=p(c(z))H(z,0)=p(c(z)), there exists a continuous L:k(Z×I)→UL:k(Z\times I)\to UL:k(Z×I)→U with πU(L(z,t))=πZ(z)\pi_U(L(z,t))=\pi_Z(z)πU​(L(z,t))=πZ​(z), L(z,0)=c(z)L(z,0)=c(z)L(z,0)=c(z), and p(L(z,t))=H(z,t)p(L(z,t))=H(z,t)p(L(z,t))=H(z,t) for all z,tz,tz,t. The second condition is the existence of a map q:V→Uq:V\to Uq:V→U over BBB and homotopies over BBB from q∘pq\circ pq∘p to idU\mathrm{id}_UidU​ and from p∘qp\circ qp∘q to idV\mathrm{id}_VidV​. Assume C(si:En→En+1)\mathcal C(s_i:E_n\to E_{n+1})C(si​:En​→En+1​) for every n≥0n\geq0n≥0 and every 0≤i≤n0\leq i\leq n0≤i≤n, where sis_isi​ is assigned to the opposite of the simplex surjection identifying iii and i+1i+1i+1, and assume C(e0:B→E0)\mathcal C(e_0:B\to E_0)C(e0​:B→E0​), where BBB has its identity projection. Then C(en:B→En)\mathcal C(e_n:B\to E_n)C(en​:B→En​) holds for every n≥0n\geq0n≥0, including n=0n=0n=0. No nonemptiness assumptions are imposed on the spaces.

Human review
  • Endorsed by Shuze Chen · Nov 1, 2026

    Confirmed by the moderator at approval.

  • Endorsed by Gohan · Nov 1, 2026

    Confirmed by the mission captain (proposal self-audit).

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