graph -- constructor for Graph

Synopsis

Usage:

G = graph(R,E)

G = graph(I)

G = graph(J)

G = graph(E)

G = graph(H)
Inputs:
- R, a polynomial ring, whose variables correspond to vertices of the hypergraph
- E, a list, a list of edges, which themselves are lists of vertices
- I, a monomial ideal, which must be square-free and quadratic, and whose generators become the edges of the graph
- J, an ideal, which must be square-free, quadratic, and monomial, and whose generators become the edges of the graph
- H, a hypergraph, to be converted to a graph. The edges in H must be of size two.
Outputs:
- G, a graph,

Description

The function graph is a constructor for Graph, a type of HyperGraph. The user can input a graph in a number of different ways, which we describe below. The information describing the graph is stored in a hash table.

For the first possibility, the user inputs a polynomial ring, which specifies the vertices of graph, and a list of the edges of the graph. The edges are represented as lists.

i1 : R = QQ[a..f];

i2 : E = {{a,b},{b,c},{c,f},{d,a},{e,c},{b,d}}

o2 = {{a, b}, {b, c}, {c, f}, {d, a}, {e, c}, {b, d}}

o2 : List

i3 : g = graph (R,E)

o3 = Graph{"edges" => {{a, b}, {b, c}, {c, f}, {a, d}, {c, e}, {b, d}}}
           "ring" => R
           "vertices" => {a, b, c, d, e, f}

o3 : Graph

As long as the edge list is not empty, the ring can be omitted. When a ring is not passed to the constructor, the underlying hypergraph takes its ring from the first variable found.

i4 : S = QQ[z_1..z_8];

i5 : E1 = {{z_1,z_2},{z_2,z_3},{z_3,z_4},{z_4,z_5},{z_5,z_6},{z_6,z_7},{z_7,z_8},{z_8,z_1}}

o5 = {{z , z }, {z , z }, {z , z }, {z , z }, {z , z }, {z , z }, {z , z },
        1   2     2   3     3   4     4   5     5   6     6   7     7   8  
     ------------------------------------------------------------------------
     {z , z }}
       8   1

o5 : List

i6 : E2 = {{z_1,z_2},{z_2,z_3}}

o6 = {{z , z }, {z , z }}
        1   2     2   3

o6 : List

i7 : g1 = graph E1

o7 = Graph{"edges" => {{z , z }, {z , z }, {z , z }, {z , z }, {z , z }, {z , z }, {z , z }, {z , z }}}
                         1   2     2   3     3   4     4   5     5   6     6   7     1   8     7   8
           "ring" => S
           "vertices" => {z , z , z , z , z , z , z , z }
                           1   2   3   4   5   6   7   8

o7 : Graph

i8 : g2 = graph E2

o8 = Graph{"edges" => {{z , z }, {z , z }}               }
                         1   2     2   3
           "ring" => S
           "vertices" => {z , z , z , z , z , z , z , z }
                           1   2   3   4   5   6   7   8

o8 : Graph

The list of edges could also be entered as a list of square-free quadratic monomials.

i9 : T = QQ[w,x,y,z];

i10 : e = {w*x,w*y,w*z,x*y,x*z,y*z}

o10 = {w*x, w*y, w*z, x*y, x*z, y*z}

o10 : List

i11 : g = graph e

o11 = Graph{"edges" => {{w, x}, {w, y}, {x, y}, {w, z}, {x, z}, {y, z}}}
            "ring" => T
            "vertices" => {w, x, y, z}

o11 : Graph

Another option for defining an graph is to use an ideal or monomialIdeal.

i12 : C = QQ[p_1..p_6];

i13 : i = monomialIdeal (p_1*p_2,p_2*p_3,p_3*p_4,p_3*p_5,p_3*p_6)

o13 = monomialIdeal (p p , p p , p p , p p , p p )
                      1 2   2 3   3 4   3 5   3 6

o13 : MonomialIdeal of C

i14 : graph i

o14 = Graph{"edges" => {{p , p }, {p , p }, {p , p }, {p , p }, {p , p }}}
                          1   2     2   3     3   4     3   5     3   6
            "ring" => C
            "vertices" => {p , p , p , p , p , p }
                            1   2   3   4   5   6

o14 : Graph

i15 : j = ideal (p_1*p_2,p_1*p_3,p_1*p_4,p_1*p_5,p_1*p_6)

o15 = ideal (p p , p p , p p , p p , p p )
              1 2   1 3   1 4   1 5   1 6

o15 : Ideal of C

i16 : graph j

o16 = Graph{"edges" => {{p , p }, {p , p }, {p , p }, {p , p }, {p , p }}}
                          1   2     1   3     1   4     1   5     1   6
            "ring" => C
            "vertices" => {p , p , p , p , p , p }
                            1   2   3   4   5   6

o16 : Graph

A graph can be made from any hypergraph whose edges are all of size two.

i17 : D = QQ[r_1..r_5];

i18 : h = hyperGraph {r_1*r_2,r_2*r_4,r_3*r_5,r_5*r_4,r_1*r_5}

o18 = HyperGraph{"edges" => {{r , r }, {r , r }, {r , r }, {r , r }, {r , r }}}
                               1   2     2   4     1   5     3   5     4   5
                 "ring" => D
                 "vertices" => {r , r , r , r , r }
                                 1   2   3   4   5

o18 : HyperGraph

i19 : g = graph h

o19 = Graph{"edges" => {{r , r }, {r , r }, {r , r }, {r , r }, {r , r }}}
                          1   2     2   4     1   5     3   5     4   5
            "ring" => D
            "vertices" => {r , r , r , r , r }
                            1   2   3   4   5

o19 : Graph

Not all graph constructors are able to make the empty graph, that is, the graph with no edges. Specifically, the constructors that take only a list cannot make the empty graph because the underlying ring is not given. To define the empty graph, give an explicit polynomial ring, or give the (monomial) ideal.

i20 : E = QQ[m,n,o,p]

o20 = E

o20 : PolynomialRing

i21 : graph(E, {})

o21 = Graph{"edges" => {}             }
            "ring" => E
            "vertices" => {m, n, o, p}

o21 : Graph

i22 : graph monomialIdeal(0_E)  -- the zero element of E (do not use 0)

o22 = Graph{"edges" => {}             }
            "ring" => E
            "vertices" => {m, n, o, p}

o22 : Graph

i23 : graph ideal(0_E)

o23 = Graph{"edges" => {}             }
            "ring" => E
            "vertices" => {m, n, o, p}

o23 : Graph

Ways to use graph:

graph(HyperGraph)
graph(Ideal)
graph(List)
graph(MonomialIdeal)
graph(PolynomialRing,List)

For the programmer

The object graph is a method function.

graph -- constructor for Graph

Synopsis

Description

See also

Ways to use graph:

For the programmer