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A directed graph is a set of nodes (or vertices) and a set of edges (disjoint from the nodes), where each edge has an initial node and a terminal node.

More formally, a directed graph is a quadruple (V, E, α, β), where

- V is a (possibly infinite) set of nodes,
- E is a set of edges, disjoint from V,
- α and β are two mappings from each edge e ∈ E, with α(e) ∈ V the origin (or initial vertex) of e and β(e) ∈ V the terminal (or end vertex) of e. α(e) and β(e) are the endpoints of e.

An undirected graph is a set of nodes (or vertices) and a set of edges (disjoint from the nodes), where each edge has two endpoints, not distinguished from each other.

More formally, an undirected graph is a triple (V, E, δ), where

- δ is a mapping from each edge e ∈ E to two nodes (not necessarily distinct) v1 ∈ V and v2 ∈ V. v1 and v2 are the endpoints of e.

Two directed graphs (V, E, α, β) and (V', E', α', β') are isomorphic if there are bijections g:V→V' and h:E→E' such that for each e∈E,

α'(h(e))=g(α(e))
and
β'(h(e))=g(β(e))

A loop is an edge both whose endpoints are equal.

A graph contains multiple edges when two edges share corresponding endpoints. If a directed graph has no multiple edges, then for it E⊆V×V.

A graph is simple if it contains neither loops nor multiple edges.

Informally, a partial graph is obtained by deleting some edges, and a subgraph is obtained by deleting some nodes and any edges that are their endpoints.

G'=(V', E', α', β') is a partial graph of G=(V, E, α, β) if

- V'=V,
- E'⊆E, and
- ∀e∈E' : α'(e) = α(e) and β'(e) = β(e)

G' is a subgraph of G if

- V'⊆V,
- E' = {e∈E | α(e)∈V' and β(e)∈V' }, and
- ∀e∈E' : α'(e) = α(e) and β'(e) = β(e)