implemented power of graph function under basic methods

src/sage/graphs/generic_graph.py

@@ -15746,6 +15746,95 @@
  weight_function=weight_function,
  check_weight=check_weight)[0]
 
+ def power(self, k):
+ r"""
+ Return the `k`-th power graph of ``self``.
+
+ In the `k`-th power graph of a graph `G`, there is an edge between
+ any pair of vertices at distance at most `k` in `G`, where the
+ distance is considered in the unweighted graph. In a directed graph,
+ there is an arc from a vertex `u` to a vertex `v` if there is a path
+ of length at most `k` in `G` from `u` to `v`.
+
+ INPUT:
+
+ - ``k`` -- integer; the maximum path length for considering edges in
+ the power graph.
+
+ OUTPUT:
+
+ - The kth power graph based on shortest distances between nodes.
+
+ EXAMPLES:
+
+ Testing on undirected graphs::
+
+ sage: G = Graph([(0, 1), (1, 2), (2, 3), (3, 0), (2, 4), (4, 5)])
+ sage: PG = G.power(2)
+ sage: PG.edges(sort=True, labels=False)
+ [(0, 1), (0, 2), (0, 3), (1, 2), (1, 3), (1, 4), (2, 3), (2, 4), (2, 5), (3, 4), (4, 5)]
+
+ Testing on directed graphs::
+
+ sage: G = DiGraph([(0, 1), (1, 2), (2, 3), (3, 0), (2, 4), (4, 5)])
+ sage: PG = G.power(3)
+ sage: PG.edges(sort=True, labels=False)
+ [(0, 1), (0, 2), (0, 3), (0, 4), (1, 0), (1, 2), (1, 3), (1, 4), (1, 5), (2, 0), (2, 1), (2, 3), (2, 4), (2, 5), (3, 0), (3, 1), (3, 2), (4, 5)]
+
+ TESTS:
+
+ Testing when k < 0::
+
+ sage: G = Graph([(0, 1), (1, 2), (2, 3), (3, 0), (2, 4), (4, 5)])
+ sage: PG = G.power(-2)
+ Traceback (most recent call last):
+ ...
+ ValueError: distance must be a non-negative integer, not -2
+
+ Testing when k = 0::
+
+ sage: G = Graph([(0, 1), (1, 2), (2, 3), (3, 0), (2, 4), (4, 5)])
+ sage: PG = G.power(0)
+ sage: PG.edges(sort=True, labels=False)
+ []
+
+ Testing when k = 1::
+
+ sage: G = Graph([(0, 1), (1, 2), (2, 3), (3, 0), (2, 4), (4, 5)])
+ sage: PG = G.power(1)
+ sage: PG.edges(sort=True, labels=False)
+ [(0, 1), (0, 3), (1, 2), (2, 3), (2, 4), (4, 5)]
+
+ Testing when k = Infinity::
+
+ sage: G = Graph([(0, 1), (1, 2), (2, 3), (3, 0), (2, 4), (4, 5)])
+ sage: PG = G.power(Infinity)
+ Traceback (most recent call last):
+ ...
+ ValueError: distance must be a non-negative integer, not +Infinity
+
+ Testing on graph with multiple edges::
+
+ sage: G = DiGraph([(0, 1), (0, 1), (1, 2), (2, 3), (3, 0), (2, 4), (4, 5)], multiedges=True)
+ sage: PG = G.power(3)
+ sage: PG.edges(sort=True, labels=False)
+ [(0, 1), (0, 2), (0, 3), (0, 4), (1, 0), (1, 2), (1, 3), (1, 4), (1, 5), (2, 0), (2, 1), (2, 3), (2, 4), (2, 5), (3, 0), (3, 1), (3, 2), (4, 5)]
+ """
+ from sage.graphs.digraph import DiGraph
+ from sage.graphs.graph import Graph
+
+ power_of_graph = DiGraph() if self.is_directed() else Graph()
+
+ for u in self:
+ for v in self.breadth_first_search(u, distance=k):
+ if u != v:
+ power_of_graph.add_edge(u, v)
+
+ if self.name():
+ power_of_graph.name("power({})".format(self.name()))
+
+ return power_of_graph
+
  def girth(self, certificate=False):
  """
  Return the girth of the graph.