Lax12.Admissibility

Admissibility

concepts/Lax12/Admissibility.lean · lax-12

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    Definition

    Fix a linear ordering of the vertices of a graph G. An admissible family of size k at a vertex v consists of k paths of length at most r that start at v, end at vertices smaller than v, and are pairwise disjoint apart from v. The r-admissibility adm_r(G) is the minimum over all orderings of the largest k + 1 for which some vertex of G carries an admissible family of size k. Counting v itself is the usual convention: it makes admissibility at least 1 and at most the strong r-coloring number.

    This is Definition 2.2 of Chapter 2 of the source lecture notes (2019/20 edition), minimized over vertex orderings as in their Definition 2.3; the notes give the same rationale for the +1, namely consistency with the reachability sets, which contain the vertex itself.

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    1import Lax12.GraphClasses
    2import Mathlib.Combinatorics.SimpleGraph.Walk.Basic
    3import Mathlib.Data.Nat.Lattice
    4
    5/-!
    6---
    7title: Admissibility
    8type: definition
    9---
    10Fix a linear ordering of the vertices of a graph *G*. An admissible
    11family of size *k* at a vertex *v* consists of *k* paths of length at
    12most *r* that start at *v*, end at vertices smaller than *v*, and are
    13pairwise disjoint apart from *v*. The *r*-admissibility adm_r(*G*) is
    14the minimum over all orderings of the largest *k* + 1 for which some
    15vertex of *G* carries an admissible family of size *k*. Counting *v*
    16itself is the usual convention: it makes admissibility at least 1 and at
    17most the strong *r*-coloring number.
    18
    19This is Definition 2.2 of Chapter 2 of the source lecture notes (2019/20
    20edition), minimized over vertex orderings as in their Definition 2.3;
    21the notes give the same rationale for the +1, namely consistency with
    22the reachability sets, which contain the vertex itself.
    23
    24# Formalization notes
    25
    26The ordering is a permutation `π` of `Fin n`, as in the coloring-number
    27concept, and the family is indexed by `Fin k`, so its size is the
    28parameter of the structure rather than a derived cardinality. Paths are
    29stated as walks, as everywhere in this submission: bypassing a walk to a
    30path shrinks its support, so a family of walks meeting only in `v`
    31yields a family of paths meeting only in `v` of the same size, and the
    32two readings define the same largest `k`.
    33
    34The endpoints of a family are automatically pairwise distinct — a shared
    35endpoint would lie on two paths and differ from `v` — so no injectivity
    36field is carried. `HasAdmAtMost G r k` says that some ordering admits no
    37family of `k` paths anywhere, and `adm` is the least such `k`. The set
    38is nonempty (`k = n + 1` always qualifies, since a family's endpoints
    39are `k` distinct vertices other than `v`), so `Nat.sInf ∅ = 0` is never
    40exercised; on the empty graph every bound holds vacuously and `adm`,
    41`wcol`, `scol` are all `0`.
    42-/
    43
    44namespace Lax12.Admissibility
    45
    46open Lax12.GraphClasses
    47
    48/-- An admissible family of `k` paths at `v` under the ordering `π`:
    49`k` walks of length at most `r` out of `v`, each ending strictly before
    50`v` in the ordering, pairwise meeting only in `v`. -/
    51structure AdmFamily {n : ℕ} (G : SimpleGraph (Fin n))
    52 (π : Equiv.Perm (Fin n)) (r k : ℕ) (v : Fin n) where
    53 /-- The endpoint of each path. -/
    54 target : Fin k → Fin n
    55 /-- The path from `v` to each endpoint. -/
    56 path : ∀ i, G.Walk v (target i)
    57 /-- Every endpoint comes strictly before `v` in the ordering. -/
    58 target_lt : ∀ i, π (target i) < π v
    59 /-- Every path has length at most `r`. -/
    60 length_le : ∀ i, (path i).length ≤ r
    61 /-- Distinct paths meet only in `v`. -/
    62 meet_eq : ∀ i j, i ≠ j → ∀ y ∈ (path i).support,
    63 y ∈ (path j).support → y = v
    64
    65/-- Some vertex ordering admits no admissible family of `k` paths at any
    66vertex: the `r`-admissibility is at most `k`. -/
    67def HasAdmAtMost {n : ℕ} (G : SimpleGraph (Fin n)) (r k : ℕ) : Prop :=
    68 ∃ π : Equiv.Perm (Fin n), ∀ (v : Fin n) (j : ℕ),
    69 Nonempty (AdmFamily G π r j v) → j + 1 ≤ k
    70
    71/-- The `r`-admissibility of `G`: the least bound achieved by some
    72vertex ordering. -/
    73noncomputable def adm {n : ℕ} (G : SimpleGraph (Fin n)) (r : ℕ) : ℕ :=
    74 sInf {k | HasAdmAtMost G r k}
    75
    76end Lax12.Admissibility
    77

    Formalization notes

    The ordering is a permutation ππ of FinnFin n, as in the coloring-number concept, and the family is indexed by FinkFin k, so its size is the parameter of the structure rather than a derived cardinality. Paths are stated as walks, as everywhere in this submission: bypassing a walk to a path shrinks its support, so a family of walks meeting only in vv yields a family of paths meeting only in vv of the same size, and the two readings define the same largest kk.

    The endpoints of a family are automatically pairwise distinct — a shared endpoint would lie on two paths and differ from vv — so no injectivity field is carried. HasAdmAtMostGrkHasAdmAtMost G r k says that some ordering admits no family of kk paths anywhere, and admadm is the least such kk. The set is nonempty (k=n+1k = n + 1 always qualifies, since a family's endpoints are kk distinct vertices other than vv), so Nat.sInf=0Nat.sInf ∅ = 0 is never exercised; on the empty graph every bound holds vacuously and admadm, wcolwcol, scolscol are all 00.

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