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- Newsgroups: comp.theory
- Path: sparky!uunet!wupost!cs.utexas.edu!torn!watserv2.uwaterloo.ca!watserv1!phonon.uwaterloo.ca!nrswart
- From: nrswart@phonon.uwaterloo.ca (Nicholas R. Swart)
- Subject: Update on Graph Isomorphism Proof
- Message-ID: <Bzs0KA.8pD@watserv1.uwaterloo.ca>
- Sender: news@watserv1.uwaterloo.ca
- Organization: University of Waterloo
- Date: Thu, 24 Dec 1992 18:26:33 GMT
- Lines: 86
-
- I'm posting this message for my father:
-
- -------------------------------------------------------------------------
-
-
- Graph Isomorphism et al
-
- Although we have not contributed to the comp.theory discussion
- directly, we would like to publicly -- in open forum -- express our
- appreciation to all those of you who have provided helpful dialogue
- on a one-on-one basis regarding our graph isomorphism and N=1
- papers. It seems appropriate, moreover, to bring you all up to
- date as to the current status of these papers. There are several
- aspects of the graph isomorphism paper and its relationship to the
- N=1 paper which drew significant attention and the outcome of the
- resulting dialogue is as follows:
-
- * Several people pointed out that corollary 1 was not fully
- justified and possibly in error. We are happy to report that all
- those who have discussed this point with us are now agreed that it
- is not in error and can be justified. Strictly speaking, however,
- it does need an additional lemma concerning the implications of the
- Ta=b constraints. The effect of these constraints is largely
- benign in that all that they do is require some of the t[abcd] and
- possibly some of the s[ab] variables to be zero. Any solution to
- formulation F3 which has a C_0 pattern or a C_0 sub-pattern, which
- avoids these zeroes, thus implies the existence of a Q matrix
- solution which likewise avoids these zeroes which, in turn, implies
- that the graphs being examined are isomorphic.
-
- * Quite a few people have suggested that even if the graph
- isomorphism paper is correct that this does not imply the
- correctness of the N=1 paper. We are happy to report that on this
- point as well there is now complete agreement with all those with
- whom we have discussed the issue. If there is an LP formulation of
- the type we have developed, which allows us to solve the graph
- isomorphism problem in polynomial time, then that very same
- formulation -- with minor modifications -- can be used to solve the
- Hamilton tour, clique, 3-dimensional matching and any other
- subgraph isomorphism problem in polynomial time as well.
- Everything thus hinges on the graph isomorphism paper.
-
- * The Tony Mansfield counterexample regarding the
- applicability of the Q matrices, with respect to ensuring
- isomorphism, is not strictly relevant since the paper was not
- designed to address the issue of multigraphs. After some
- electronic dialogue, Tony Mansfield himself has suggested an
- elegant way of coping with multigraphs or weighted graphs entirely
- within the spirit of the Ta=b constraints. It is thus possible to
- handle directed or undirected graphs, graphs with loops,
- multigraphs, weighted graphs and subgraphs -- all within the same
- general framework.
-
- * With respect to the Michael Robson counterexample it needs
- to be pointed out that although the example he gives does not have
- a C_0 pattern it does have a C_0 sub-pattern since all the vectors
- q5 through to q10 and any convex combination of them provide such
- C_0 sub-patterns. An example of this kind does not therefore
- invalidate the proper functioning of formulation F4. Nevertheless,
- the existence of solutions of this type necessitate some
- modifications to the theorem and/or additions to the formulation.
- There is at least one way of making the necessary changes and we
- will be reporting on this in due course.
-
- We are particularly in the debt of William Naylor for
- suggesting that our LP approach using the Q matrices ought to be
- taken seriously even if it needs some modification. We are further
- able to report that our experimental checks of formulation F4 have
- turned up no cases of failure -- neither with respect to graph
- isomorphism nor with respect to Hamiltonicity checking. Moreover,
- we have found that a form of Boolean closure is a very affective
- method for reducing the need to resort to actual linear
- programming. We are in the process of creating an improved version
- of the isomorphism paper which takes into account all the above
- points and will make it available when it is ready.
-
- .. E.R. Swart & S.J. Gismondi ..
-
- ---------------------------------------------------------------------
-
- Nicholas R. Swart
- Dept. of Elec. and Comp. Engineering
- University of Waterloo
- Waterloo, Ontario
- CANADA
- N2L 3G1
-