Tuesday, 25 March 2008

oc.optimization control - Is there an "adjacency matrix" for weighted directed graphs that captures the weights?

I am currently writing up some notes on the max-plus algebra mathbbRmax (for those that have never seen the term "max-plus algebra", it is just mathbbR with addition replaced by max and multiplication replaced by addition. For some reason, authors whose main interest is control-theoretic applications never seem to use the term "tropical", and I have been reading from such authors). There is a nice result which says the following:



textbfTheorem. Let G be a directed graph on n vertices such that each arc (i,j) in G has a real weight w(i,j). Define the ntimesn matrix A by (A)ij=w(i,j) if (i,j) is an arc, and (A)ij=infty otherwise. Then for each k>0, the maximum weight of a path of length k from vertex i to vertex j is given by (Aotimesk)ij (here, Aotimesk is just the kth power of A, computed using the mathbbRmax operations).



This result is certainly analagous to the standard result that the ij-entry of the kth power of the adjacency matrix gives the number of walks of length k from vertex i to vertex j. When writing up my notes I found myself claiming that the above theorem provides some evidence that mathbbRmax is in fact a natural setting in which to study weighted digraphs, since there is no natural definition of an ``adjacency matrix'' of a weighted digraph (in the usual setting of mathbbRntimesn) that gives useful information about the weights. This seemed like too strong of a claim, especially since I am no expert in networks or combinatorial optimization. This leads to the question:



textbfQuestion. Is there a standard matrix (in mathbbRntimesn) associated with a weighted digraph that is analogous to the adjacency matrix and captures in a useful way the weights of the arcs?



textbfClarification: By "analogous to the adjacency matrix" I mean a matrix that is defined simply in terms of the graph (vertices, arcs, and weights). I imagine there are all sorts of matrices associated to weighted digraphs so that computers can be used to analyze networks. But I am not interested in, say, a matrix that requires a complicated algorithm to compute its entries.

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