Undirected graphs

Content created by Egbert Rijke, Fredrik Bakke, Jonathan Prieto-Cubides, Julian KG, fernabnor, louismntnu and Szumi Xie.

Created on 2022-02-10.
Last modified on 2023-09-11.

module graph-theory.undirected-graphs where
open import foundation.cartesian-product-types
open import foundation.dependent-pair-types
open import foundation.equivalences
open import foundation.transport-along-identifications
open import foundation.universe-levels
open import foundation.unordered-pairs

open import graph-theory.directed-graphs


An undirected graph consists of a type V of vertices and a family E of types over the unordered pairs of V.


Undirected-Graph : (l1 l2 : Level)  UU (lsuc l1  lsuc l2)
Undirected-Graph l1 l2 = Σ (UU l1)  V  unordered-pair V  UU l2)

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

  vertex-Undirected-Graph : UU l1
  vertex-Undirected-Graph = pr1 G

  unordered-pair-vertices-Undirected-Graph : UU (lsuc lzero  l1)
  unordered-pair-vertices-Undirected-Graph =
    unordered-pair vertex-Undirected-Graph

  type-unordered-pair-vertices-Undirected-Graph :
    unordered-pair-vertices-Undirected-Graph  UU lzero
  type-unordered-pair-vertices-Undirected-Graph p = type-unordered-pair p

  element-unordered-pair-vertices-Undirected-Graph :
    (p : unordered-pair-vertices-Undirected-Graph) 
    type-unordered-pair-vertices-Undirected-Graph p  vertex-Undirected-Graph
  element-unordered-pair-vertices-Undirected-Graph p = element-unordered-pair p

  edge-Undirected-Graph : unordered-pair-vertices-Undirected-Graph  UU l2
  edge-Undirected-Graph = pr2 G

  total-edge-Undirected-Graph : UU (lsuc lzero  l1  l2)
  total-edge-Undirected-Graph =
    Σ unordered-pair-vertices-Undirected-Graph edge-Undirected-Graph

The forgetful functor from directed graphs to undirected graphs

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

  undirected-graph-Graph : Undirected-Graph l1 l2
  pr1 undirected-graph-Graph = vertex-Directed-Graph G
  pr2 undirected-graph-Graph p =
    Σ ( type-unordered-pair p)
      ( λ x 
        Σ ( type-unordered-pair p)
          ( λ y 
            edge-Directed-Graph G
              ( element-unordered-pair p x)
              ( element-unordered-pair p y)))

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

  graph-Undirected-Graph : Directed-Graph l1 (lsuc lzero  l1  l2)
  pr1 graph-Undirected-Graph = vertex-Undirected-Graph G
  pr2 graph-Undirected-Graph x y =
    Σ ( unordered-pair-vertices-Undirected-Graph G)
      ( λ p 
        ( mere-Eq-unordered-pair (standard-unordered-pair x y) p) ×
        ( edge-Undirected-Graph G p))

  graph-Undirected-Graph' : Directed-Graph l1 l2
  pr1 graph-Undirected-Graph' = vertex-Undirected-Graph G
  pr2 graph-Undirected-Graph' x y =
    edge-Undirected-Graph G (standard-unordered-pair x y)

Transporting edges along equalities of unordered pairs of vertices

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

  equiv-tr-edge-Undirected-Graph :
    (p q : unordered-pair-vertices-Undirected-Graph G)
    (α : Eq-unordered-pair p q) 
    edge-Undirected-Graph G p  edge-Undirected-Graph G q
  equiv-tr-edge-Undirected-Graph p q α =
    equiv-tr (edge-Undirected-Graph G) (eq-Eq-unordered-pair' p q α)

  tr-edge-Undirected-Graph :
    (p q : unordered-pair-vertices-Undirected-Graph G)
    (α : Eq-unordered-pair p q) 
    edge-Undirected-Graph G p  edge-Undirected-Graph G q
  tr-edge-Undirected-Graph p q α =
    tr (edge-Undirected-Graph G) (eq-Eq-unordered-pair' p q α)

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