Trac #30159: Adding new small graph structures

URL: https://trac.sagemath.org/30159
Reported by: gh-Fightlapa
Ticket author(s): Jakub Jabłoński
Reviewer(s): David Coudert

src/sage/graphs/generators/basic.py

@@ -608,6 +608,67 @@ def DiamondGraph():
     edges = [(0, 1), (0, 2), (1, 2), (1, 3), (2, 3)]
     return graph.Graph(edges, pos=pos_dict, name="Diamond Graph")
 
+def GemGraph():
+    """
+    Return a gem graph with 5 nodes.
+
+    A gem graph is a fan graph (4,1).
+
+    PLOTTING: Upon construction, the position dictionary is filled to
+    override the spring-layout algorithm. By convention, the gem
+    graph is drawn as a gem, with the sharp part on the bottom.
+
+    EXAMPLES:
+
+    Construct and show a gem graph::
+
+        sage: g = graphs.GemGraph()
+        sage: g.show() # long time
+    """
+    pos_dict = {0:(0.5,0),1:(0,0.75),2:(0.25,1),3:(0.75,1),4:(1,0.75)}
+    edges = [(0, 1), (0, 2), (0, 3), (0, 4), (1, 2), (2, 3), (3, 4)]
+    return graph.Graph(edges, pos=pos_dict, name="Gem Graph")
+
+def ForkGraph():
+    """
+    Return a fork graph with 5 nodes.
+
+    A fork graph, sometimes also called chair graph, is 5 vertex tree.
+
+    PLOTTING: Upon construction, the position dictionary is filled to
+    override the spring-layout algorithm. By convention, the fork
+    graph is drawn as a fork, with the sharp part on the bottom.
+
+    EXAMPLES:
+
+    Construct and show a fork graph::
+
+        sage: g = graphs.ForkGraph()
+        sage: g.show() # long time
+    """
+    pos_dict = {0:(0,0),1:(1,0),2:(0,1),3:(1,1),4:(0,2)}
+    edges = [(0, 2), (2, 3), (3, 1), (2, 4)]
+    return graph.Graph(edges, pos=pos_dict, name="Fork Graph")
+
+def DartGraph():
+    """
+    Return a dart graph with 5 nodes.
+
+    PLOTTING: Upon construction, the position dictionary is filled to
+    override the spring-layout algorithm. By convention, the dart
+    graph is drawn as a dart, with the sharp part on the bottom.
+
+    EXAMPLES:
+
+    Construct and show a dart graph::
+
+        sage: g = graphs.DartGraph()
+        sage: g.show() # long time
+    """
+    pos_dict = {0:(0,1),1:(-1,0),2:(1,0),3:(0,-1),4:(0,0)}
+    edges = [(0, 1), (0, 2), (1, 4), (2, 4), (0, 4), (3, 4)]
+    return graph.Graph(edges, pos=pos_dict, name="Dart Graph")
+
 def EmptyGraph():
     """
     Returns an empty graph (0 nodes and 0 edges).

src/sage/graphs/graph_generators.py

@@ -67,6 +67,9 @@ def __append_to_doc(methods):
      "CompleteGraph",
      "CompleteMultipartiteGraph",
      "DiamondGraph",
+     "GemGraph",
+     "DartGraph",
+     "ForkGraph",
      "DipoleGraph",
      "EmptyGraph",
      "Grid2dGraph",
@@ -1893,6 +1896,9 @@ def quadrangulations(self, order, minimum_degree=None, minimum_connectivity=None
     CompleteBipartiteGraph   = staticmethod(basic.CompleteBipartiteGraph)
     CompleteMultipartiteGraph= staticmethod(basic.CompleteMultipartiteGraph)
     DiamondGraph             = staticmethod(basic.DiamondGraph)
+    GemGraph                 = staticmethod(basic.GemGraph)
+    DartGraph                = staticmethod(basic.DartGraph)
+    ForkGraph                = staticmethod(basic.ForkGraph)
     EmptyGraph               = staticmethod(basic.EmptyGraph)
     Grid2dGraph              = staticmethod(basic.Grid2dGraph)
     GridGraph                = staticmethod(basic.GridGraph)

src/sage/graphs/graph_list.py

@@ -303,6 +303,9 @@ def show_graphs(graph_list, **kwds):
         sage: glist.append(graphs.BarbellGraph(7, 4))
         sage: glist.append(graphs.CycleGraph(15))
         sage: glist.append(graphs.DiamondGraph())
+        sage: glist.append(graphs.GemGraph())
+        sage: glist.append(graphs.DartGraph())
+        sage: glist.append(graphs.ForkGraph())
         sage: glist.append(graphs.HouseGraph())
         sage: glist.append(graphs.HouseXGraph())
         sage: glist.append(graphs.KrackhardtKiteGraph())
@@ -316,7 +319,7 @@ def show_graphs(graph_list, **kwds):
     Check that length is <= 20::
 
         sage: len(glist)
-        14
+        17
 
     Show the graphs in a graphics array::