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The holonomy functor is faithful

Proved
GaugeFunctor.holF_faithful

by Alien60 · Oct 9, 2026 · Mathlib 0df444a (Lean v4.33.1)

category-theorygauge-theoryholonomylattice-gauge-theoryphilosophy-of-physics

Setting: XXX is a finite directed graph with vertex set V={0,…,nV−1}V=\{0,\dots,n_V-1\}V={0,…,nV​−1}, edge set EEE and maps src,tgt:E→V\mathrm{src},\mathrm{tgt}:E\to Vsrc,tgt:E→V; a walk may use edges in either direction, and XXX is connected from xxx if every vertex is reached by a walk from xxx. GGG is any group. A connection is A:E→GA:E\to GA:E→G; its holonomy holA(w)\mathrm{hol}_A(w)holA​(w) along a walk www is the ordered product of A(e)A(e)A(e) on forward steps and A(e)−1A(e)^{-1}A(e)−1 on backward steps. A gauge transformation g:V→Gg:V\to Gg:V→G acts by (g⋅A)(e)=g(src e) A(e) g(tgt e)−1(g\cdot A)(e)=g(\mathrm{src}\,e)\,A(e)\,g(\mathrm{tgt}\,e)^{-1}(g⋅A)(e)=g(srce)A(e)g(tgte)−1. PotG(X)\mathsf{Pot}_G(X)PotG​(X) is the category whose objects are connections and whose arrows A→BA\to BA→B are gauge transformations ggg with g⋅A=Bg\cdot A=Bg⋅A=B. Holx\mathsf{Hol}_xHolx​ is the category of holonomy maps at xxx (assignments HHH of group elements to walks, multiplicative on loops at xxx and unchanged by inserting a backtrack into a loop at xxx), with arrows h:H→H′h:H\to H'h:H→H′ the elements h∈Gh\in Gh∈G such that H′(γ)=h H(γ) h−1H'(\gamma)=h\,H(\gamma)\,h^{-1}H′(γ)=hH(γ)h−1 for every loop γ\gammaγ at xxx. The holonomy functor holx:PotG(X)→Holx\mathrm{hol}_x:\mathsf{Pot}_G(X)\to\mathsf{Hol}_xholx​:PotG​(X)→Holx​ sends A↦holAA\mapsto\mathrm{hol}_AA↦holA​ and g↦g(x)g\mapsto g(x)g↦g(x).

Statement. If XXX is connected from xxx, then holx\mathrm{hol}_xholx​ is faithful: two gauge arrows g,g′:A→Bg,g':A\to Bg,g′:A→B with g(x)=g′(x)g(x)=g'(x)g(x)=g′(x) are equal.

Preamble
import Mathlib
import Definitions.Def_GaugeFunctor_defs

open CategoryTheory
Formal statement
namespace GaugeFunctor

/-- On a graph connected from `x`, a gauge transformation relating two connections is
determined by its value at `x`: the holonomy functor is faithful. -/
theorem holF_faithful (X : Graph) (G : Type) [Group G] (x : Fin X.nV)
    (hX : X.Connected x) : (holF X G x).Faithful := by sorry

end GaugeFunctor
Source
S. Rosenstock, J. O. Weatherall, A categorical equivalence between generalized holonomy maps on a connected manifold and principal connections on bundles over that manifold, J. Math. Phys. 57 (2016) 102902, https://arxiv.org/abs/1504.02401, p. 3 (Barrett representation theorem, uniqueness) and Theorem 2. Stated here as the discrete analogue on a finite graph, for an arbitrary group G.
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What the Lean code literally says, in plain math · claude-opus-5-5

Let XXX be a graph, GGG a group, xxx a vertex with XXX connected from xxx. Then the holonomy functor holFx\mathrm{holF}_xholFx​ is faithful: for any connections A,BA,BA,B and gauge transformations g,g′g,g'g,g′ with g⋅A=B=g′⋅Ag\cdot A=B=g'\cdot Ag⋅A=B=g′⋅A, if g(x)=g′(x)g(x)=g'(x)g(x)=g′(x) then g=g′g=g'g=g′ (as functions on all vertices).

Notes: plausibly true via holg⋅A(w)=g(x) holA(w) g(y)−1\mathrm{hol}_{g\cdot A}(w)=g(x)\,\mathrm{hol}_A(w)\,g(y)^{-1}holg⋅A​(w)=g(x)holA​(w)g(y)−1 along a walk w:x→yw:x\to yw:x→y. Connectivity is needed (false for an isolated vertex and nontrivial GGG). Docstring matches.

Human review
  • Endorsed by Shuze Chen · Oct 11, 2026

    Confirmed by the moderator at approval.

  • Endorsed by Alien60 · Oct 11, 2026

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

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