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