Incidence in undirected graphs

Content created by Egbert Rijke, Fredrik Bakke and Jonathan Prieto-Cubides.

Created on 2022-08-14.
Last modified on 2024-10-16.

module graph-theory.neighbors-undirected-graphs where
Imports 
open import foundation.dependent-pair-types
open import foundation.equality-dependent-pair-types
open import foundation.equivalences
open import foundation.function-types
open import foundation.functoriality-dependent-pair-types
open import foundation.homotopies
open import foundation.identity-types
open import foundation.universe-levels
open import foundation.unordered-pairs

open import graph-theory.equivalences-undirected-graphs
open import graph-theory.undirected-graphs

Idea

The type of neighbors a vertex x of an undirected graph G is the type of all vertices y in G equipped with an edge from x to y.

Definition

module _
  {l1 l2 : Level} (G : Undirected-Graph l1 l2)
  where

  neighbor-Undirected-Graph : vertex-Undirected-Graph G → UU (l1 ⊔ l2)
  neighbor-Undirected-Graph x =
    Σ ( vertex-Undirected-Graph G)
      ( λ y → edge-Undirected-Graph G (standard-unordered-pair x y))

Properties

Equivalences of undirected graphs induce equivalences on types of neighbors

module _
  {l1 l2 l3 l4 : Level}
  (G : Undirected-Graph l1 l2) (H : Undirected-Graph l3 l4)
  where

  equiv-neighbor-equiv-Undirected-Graph :
    (e : equiv-Undirected-Graph G H) (x : vertex-Undirected-Graph G) →
    neighbor-Undirected-Graph G x ≃
    neighbor-Undirected-Graph H (vertex-equiv-Undirected-Graph G H e x)
  equiv-neighbor-equiv-Undirected-Graph e x =
    equiv-Σ
      ( λ y →
        edge-Undirected-Graph H
          ( standard-unordered-pair (vertex-equiv-Undirected-Graph G H e x) y))
      ( equiv-vertex-equiv-Undirected-Graph G H e)
      ( equiv-edge-standard-unordered-pair-vertices-equiv-Undirected-Graph
          G H e x)

  neighbor-equiv-Undirected-Graph :
    (e : equiv-Undirected-Graph G H) (x : vertex-Undirected-Graph G) →
    neighbor-Undirected-Graph G x →
    neighbor-Undirected-Graph H (vertex-equiv-Undirected-Graph G H e x)
  neighbor-equiv-Undirected-Graph e x =
    map-equiv (equiv-neighbor-equiv-Undirected-Graph e x)

neighbor-id-equiv-Undirected-Graph :
  {l1 l2 : Level}
  (G : Undirected-Graph l1 l2) (x : vertex-Undirected-Graph G) →
  neighbor-equiv-Undirected-Graph G G (id-equiv-Undirected-Graph G) x ~ id
neighbor-id-equiv-Undirected-Graph G x (pair y e) =
  eq-pair-eq-fiber
    ( edge-standard-unordered-pair-vertices-id-equiv-Undirected-Graph G x y e)

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