graph theory:
    Graph is a basic structure in graph theory. It is a network of vertices and curved edges from some vertices to others or to themselves.
    
    Graphs are frequently used to model a binary relationship between the objects in same domain. For example: shops and stores with supply routes between them, computers in computer networks, ...
    
    If graph is connected graph, it has only one component (itself).
    
    Other meaning of graph is one kind (or type) of graph called simple graph or simplicial graph (simplicial 1-complex with 0D and 1D cells):
    Graph G is pair G = (V, E) of sets.
    
    
    
    Elements of set V are called vertices (nodes, points) of graph G. Vertex set of a graph G is referred to as V(G).
    Elements of set E are called edges (lines) and they are two-element subsets of set V. The edge set of a graph G is referred to as E(G).
    
    Simple graph has no loops (self-loops) or multiple edges. So its girth is at least 3.

    Also the vertices and edges of a simple graph in `RR^3` are 0-simplexes and 1-simplexes.
    
    Denotations:
    G = (V, E), where V = {1, 2, 3, 4} and E = {{1, 2,}, {1, 3}, {1, 4}, {2, 4}, {3, 4}, {4, 4}}
    
    Word simplicial is usually used in topological context and simple in nontopological.