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Diffstat (limited to '.venv/lib/python3.12/site-packages/networkx/generators/mycielski.py')
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diff --git a/.venv/lib/python3.12/site-packages/networkx/generators/mycielski.py b/.venv/lib/python3.12/site-packages/networkx/generators/mycielski.py new file mode 100644 index 00000000..804b9036 --- /dev/null +++ b/.venv/lib/python3.12/site-packages/networkx/generators/mycielski.py @@ -0,0 +1,110 @@ +"""Functions related to the Mycielski Operation and the Mycielskian family +of graphs. + +""" + +import networkx as nx +from networkx.utils import not_implemented_for + +__all__ = ["mycielskian", "mycielski_graph"] + + +@not_implemented_for("directed") +@not_implemented_for("multigraph") +@nx._dispatchable(returns_graph=True) +def mycielskian(G, iterations=1): + r"""Returns the Mycielskian of a simple, undirected graph G + + The Mycielskian of graph preserves a graph's triangle free + property while increasing the chromatic number by 1. + + The Mycielski Operation on a graph, :math:`G=(V, E)`, constructs a new + graph with :math:`2|V| + 1` nodes and :math:`3|E| + |V|` edges. + + The construction is as follows: + + Let :math:`V = {0, ..., n-1}`. Construct another vertex set + :math:`U = {n, ..., 2n}` and a vertex, `w`. + Construct a new graph, `M`, with vertices :math:`U \bigcup V \bigcup w`. + For edges, :math:`(u, v) \in E` add edges :math:`(u, v), (u, v + n)`, and + :math:`(u + n, v)` to M. Finally, for all vertices :math:`u \in U`, add + edge :math:`(u, w)` to M. + + The Mycielski Operation can be done multiple times by repeating the above + process iteratively. + + More information can be found at https://en.wikipedia.org/wiki/Mycielskian + + Parameters + ---------- + G : graph + A simple, undirected NetworkX graph + iterations : int + The number of iterations of the Mycielski operation to + perform on G. Defaults to 1. Must be a non-negative integer. + + Returns + ------- + M : graph + The Mycielskian of G after the specified number of iterations. + + Notes + ----- + Graph, node, and edge data are not necessarily propagated to the new graph. + + """ + + M = nx.convert_node_labels_to_integers(G) + + for i in range(iterations): + n = M.number_of_nodes() + M.add_nodes_from(range(n, 2 * n)) + old_edges = list(M.edges()) + M.add_edges_from((u, v + n) for u, v in old_edges) + M.add_edges_from((u + n, v) for u, v in old_edges) + M.add_node(2 * n) + M.add_edges_from((u + n, 2 * n) for u in range(n)) + + return M + + +@nx._dispatchable(graphs=None, returns_graph=True) +def mycielski_graph(n): + """Generator for the n_th Mycielski Graph. + + The Mycielski family of graphs is an infinite set of graphs. + :math:`M_1` is the singleton graph, :math:`M_2` is two vertices with an + edge, and, for :math:`i > 2`, :math:`M_i` is the Mycielskian of + :math:`M_{i-1}`. + + More information can be found at + http://mathworld.wolfram.com/MycielskiGraph.html + + Parameters + ---------- + n : int + The desired Mycielski Graph. + + Returns + ------- + M : graph + The n_th Mycielski Graph + + Notes + ----- + The first graph in the Mycielski sequence is the singleton graph. + The Mycielskian of this graph is not the :math:`P_2` graph, but rather the + :math:`P_2` graph with an extra, isolated vertex. The second Mycielski + graph is the :math:`P_2` graph, so the first two are hard coded. + The remaining graphs are generated using the Mycielski operation. + + """ + + if n < 1: + raise nx.NetworkXError("must satisfy n >= 1") + + if n == 1: + return nx.empty_graph(1) + + else: + return mycielskian(nx.path_graph(2), n - 2) |