A graph G = (L, S) given as such, a sub-graph of G is the graph G

_{1}= (L_{1}, S_{1}) formed by a subset S_{1}of vertices of G and a sub-set L_{1}of edges of L.### Example

Consider the graph G below, defined by G = (L, S), where L = {a, b, c, d, e, g, f, g, h, m, n} and S = {A, B, C, D, E, F}.

The graph G_{1} = (L_{1}, S_{1}), such as L_{1} = {a, b, g, n} and S_{1} = {A, B, C, E}, is a sub-graph of G.