ASKSAGE: Sage Q&A Forum - RSS feedhttps://ask.sagemath.org/questions/Q&A Forum for SageenCopyright Sage, 2010. Some rights reserved under creative commons license.Fri, 10 Jun 2016 15:01:31 +0200Finding number of strongly connected componentshttps://ask.sagemath.org/question/33735/finding-number-of-strongly-connected-components/I need to find the number of strongly connected components of a digraph, and I looked at the documentation for the ~DiGraph.strongly_connected_components() method. Tabbing .strongly_connected_components? gives the documentation from sage/graphs/base/static_sparse_graph.pyx for tarjan_strongly_connected_components(G) that says,
This routine returns a pair ``[nscc, scc]``, where ``nscc`` is the number of
SCCs and ``scc`` is a dictionary associating to each vertex ``v`` an
integer between ``0`` and ``nscc-1``, corresponding to the SCC containing
``v``. SCCs
are numbered in reverse topological order, that is, if ``(v,w)`` is an edge
in the graph, ``scc[v] <= scc[w]``
Which would be great, as I just need the result of tarjan_strongly_connected_components_C. However, 1) this is only called during tarjan_strongly_connected_components, which does the (admittedly minor) additional work of topologically sorting SCCs and 2) the examples (and testing) show the output of ~DiGraph.strongly_connected_components() is a list of vertex lists for each connected component. This is perfectly reasonable and clear, but more than I want, particularly as there's some work in the back end that determines what I'm looking for earlier in the process.
1) Should I just use len(~DiGraph.strongly_connected_components())?
2) Am I badly misreading the documentation for this method?
I'm in Sage 7.2.
Thanks in advance for help here!JEFLSUFri, 10 Jun 2016 15:01:31 +0200https://ask.sagemath.org/question/33735/hasse diagram of a subset of a posethttps://ask.sagemath.org/question/29791/hasse-diagram-of-a-subset-of-a-poset/E = {1,2,3}
P = SetPartitions(E)
This gives the set of all partitions of E,.
I have a subset Q of P and I want to construct the directed graph whose vertex set is this set Q and we draw an arrow from p to q in this graph if q covers p. How to construct this graph for a given P and Q?
The definition of covering relation can be found here : https://en.wikipedia.org/wiki/Covering_relation
Thanks for your valuable timing.GA316Tue, 06 Oct 2015 15:39:38 +0200https://ask.sagemath.org/question/29791/