# Synchronization and graph endomorphisms

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1 Synchronization and graph endomorphisms Peter J Cameron University of St Andrews / Queen Mary University of London 44th Southeastern Conference on Combinatorics, Graph Theory and Computing 8 March 2013

2 St Andrews Synchronization As from last Friday, I am a part-time Professor of Mathematics and Statistics at the University of St Andrews It was during a previous visit to St Andrews that the connection between synchronization and graph endomorphisms came to me during a sleepless night A few combinatorial problems which all have something in common Do there exist analogues of the Erdős Ko Rado and Baranyai s over fields with more than one element, that is, with vector space and subspace replacing set and subset? (The analogue of Baranyai would ask for a partition of the k-dimensional subspaces of an n-dimensional vector space into spreads, each spread containing every non-zero vector once) Which polar spaces have ovoids, spreads, or partitions into ovoids? For which n can we partition the k-element subsets of an n-set into Steiner systems S(3, 4, n), or into Steiner systems S(2, 4, n)? I will describe a property of permutation groups called synchronization It turns out that synchronizing implies primitive (and even basic, in terms of the O Nan Scott classification) But deciding which basic primitive groups are synchronizing usually involves almost no group theory, and turns into a combinatorial problem, usually an interesting (and hard) problem So this machine gives us a big supply of interesting combinatorial problems, old and new: the three above are examples Automata Automaton here means finite deterministic automaton An automaton is a device which can be in any one of a set Ω of internal states On the console there are a number of coloured buttons; pressing a button forces the automaton to undergo a transition, a function from Ω to itself Thus we can regard an automaton as an edge-coloured directed graph on Ω, with the property that there is a unique edge of each colour leaving each vertex An automaton is synchronizing if there is a sequence of transitions which brings it into a fixed state α Ω, from any initial state The dungeon You are in a dungeon consisting of a number of rooms Passages are marked with coloured arrows Each room contains a special door; in one room, the door leads to freedom, but in all the others, to instant death You have a schematic map of the dungeon, but you do not know where you are You can check that (Blue, Red, Blue, Blue) takes you to room 3 no matter where you start

3 Algebraic formulation Applications Multiple button presses correspond to composition of transitions The set of all functions generated by the given set S of transitions is closed under composition and contains the identity; thus it is a transformation monoid, the monoid generated by S Note that any permutation in the monoid generated by S actually lies in the group generated by the permutations in S, since a product including a non-permutation cannot be a permutation An automaton is synchronizing if and only if this monoid contains a constant function (an element of rank 1) A word in the generators (that is, a series of button presses which evaluates to a constant function) is called a reset word Industrial robotics: pieces arrive to be assembled by a robot The orientation is critical You could equip the robot with vision sensors and manipulators so that it can rotate the pieces into the correct orientation But it is much cheaper and less error-prone to regard the possible orientations of the pieces as states of an automaton on which transitions can be performed by simple machinery, and apply a reset word before the pieces arrive at the robot Bioinformatics: If a soup of DNA molecules is to perform some computation, we need the molecules to be all in a known state first We can simultaneously apply a reset word to all of them, where the transitions are induced by some chemical or biological process The Černý conjecture An example The study of synchronizing automata has been driven by the Černý conjecture, made in the 1960s and still open: Conjecture If an n-state automaton is synchronizing, then it has a reset word of length at most (n 1) 2 If true, this would be best possible, as the example on the next slide shows The Černý conjecture has been proved in a few special cases, but the best general bound is cubic in n B R R R B R R R B So BRRRBRRRB is a reset word of length 9 = (4 1) 2 It can be checked that this is the shortest reset word Replacing the square with a regular n-gon gives examples meeting the bound for all n Graph homomorphisms Graph homomorphisms From now on, graphs are loopless, without multiple edges, and undirected We let ω(x), χ(x) and α(x) be the clique number (order of the largest clique), chromatic number (smallest number of colours in a proper coloring), and independence number (order of the largest null subgraph, or clique number of the complement) in X A homomorphism from a graph X to a graph Y is a map from the vertex set of X to the vertex set of Y which maps edges to edges (A non-edge could map to a non-edge, or to an edge, or collapse to a single vertex) Two graphs X and Y are hom-equivalent if there are homomorphisms from X to Y and from Y to X

4 Homomorphisms and colorings Proposition There is a homomorphism from K m to X if and only if ω(x) m There is a homomorphism from X to K m if and only if χ(x) m A graph X is hom-equivalent to a complete graph if and only if ω(x) = χ(x) The core of a graph is the smallest graph hom-equivalent to it The third condition in the proposition is equivalent to saying that the core of X is a complete graph Graphs and transformation monoids A transformation monoid on a set Ω is a collection of maps from Ω to itself which is closed under composition and contains the identity map For example, the endomorphisms of a graph, or of any structure, on X form a transformation monoid There is a strong connection between endomorphism monoids and graphs: Any graph X has an endomorphism monoid End(X) Given a transformation monoid S, define a graph Gr(S) by the rule that v and w are joined if and only if there is no element s M mapping v and w to the same place Proposition For any transformation monoid M or graph X, M End(Gr(M)), and X Gr(End(X)); Gr(End(Gr(M)) = Gr(M) The obstruction to synchronization A transformation monoid M on Ω is non-synchronizing if and only if there is a non-null graph X, with vertex set Ω, for which ω(x) = χ(x) and M End(X) Proof If there is such a graph X, then no edge of X is collapsed by M, and M is non-synchronizing Conversely, suppose that M is non-synchronizing, and let X = Gr(M) and s an element of M of minimum rank Then M End(X), by the result on the previous slide No two points in the image of s can be collapsed by any element t of M (or st would have smaller rank than s); so the image is a clique in X Now s is a colouring, since edges map to distinct points; and clearly the size of the clique and the number of colours are both equal to the rank of s Random synchronization Conjecture The probability that the monoid generated by two random transformations of an n-set is synchronizing tends to 1 as n A pair of transformations which do not generate a synchronizing monoid must be contained in a maximal non-synchronizing monoid So we need to describe these objects By the theorem, they are full endomorphism monoids of certain graphs I have a structural characterisation of these graphs, but haven t (yet!) been able to convert this in to bounds for the number of them and the orders of their endomorphism monoids Permutation groups Permutation groups I now turn to a case that has received a lot of attention, where M is generated by a permutation group G (a subgroup of the symmetric group on Ω) together with a single non-permutation Let G be a permutation group on Ω We say that G is transitive if any element of Ω can be mapped by any other by some element of G, that is, there is no non-trivial G-invariant subset of Ω; primitive if there is no non-trivial G-invariant equivalence relation on Ω; 2-transitive if it acts transitively on the set of pairs of distinct elements of Ω, that is, there is no non-trivial binary relation on Ω A set or relation is trivial if it is invariant under the symmetric group

5 Synchronizing groups Synchronizing groups, 2 By abuse of language, we call the permutation group G synchronizing if, for any non-permutation s, the monoid G, s contains an element of rank 1 Now the main question is: Problem Which permutation groups are synchronizing? This approach was introduced by João Araújo and Ben Steinberg They hoped initially that knowledge of groups would settle some more cases of the Černý conjecture; this has not happened, but many interesting developments have A permutation group G on Ω is non-synchronizing if and only if there is a non-trivial G-invariant graph on the vertex set Ω with clique number equal to chromatic number The forward implication is immediate from the preceding theorem Conversely, if X is a G-invariant graph with clique number and chromatic number r > 1, and s is an r-colouring of X with values in an r-clique, then s is an endomorphism of X, and so G, s End(X) Synchronizing groups, 3 An algorithm It follows immediately from the theorem that a 2-transitive group is synchronizing (since it preserves no non-trivial graph at all), and a synchronizing group is primitive (since an imprimitive group preserves a complete multipartite graph) Neither implication reverses Since the Classification of Finite Simple Groups, we know much more about primitive groups Can we use this knowledge to learn more about synchronization? Given a permutation group G on Ω, is it synchronizing? Both primitivity and 2-transitivity can be tested in polynomial time, so we may assume that G is primitive but not 2-transitive Compute the non-trivial G-invariant graphs There are 2 r 2 of these, where r is the number of G-orbits on 2-sets This is potentially exponentially large, but for many interesting groups r is much smaller than n For each such graph, check whether clique number is equal to chromatic number If we find one, G is non-synchronizing; otherwise it is synchronizing Of course, clique number and chromatic number are hard in general, but we have highly symmetric graphs here, which shortens the calculation An example Consider the symmetric group S m acting on the set Ω of 2-element subsets of {1,, m}, with Ω = n = ( m 2 ) This group is primitive if m 5 There are two non-trivial G-invariant graphs: the line graph of K m, which has clique number m 1 and chromatic number m 1 if m is even, or m if m is odd; the Kneser graph (complement of the above), which has clique number m/2 and chromatic number m 3 So this group is synchronizing if and only if m is odd Combinatorics

6 Combinatorics of synchronization Groups synchronizing non-uniform maps There has been a bit of work on deciding which primitive groups are synchronizing It almost always turns out to depend on hard combinatorial problems The three introductory problems arise in considering the groups the general linear group GL(n, q), acting on the set of k-dimensional subspaces of the n-dimensional vector space; classical groups acting on the associated polar spaces; the symmetric group S n acting on 4-subsets of {1,, n} The kernel type of a map s on Ω is the partition of n giving the sizes of the inverse images of points in the image of s The map is uniform if all parts of the kernel type are equal, and non-uniform otherwise Araújo made the following conjecture: Conjecture A primitive group synchronizes every non-uniform map I will now describe some recent results on this using the graph endomorphism technique Rystsov s theorem Proof of the An early result on synchronization was proved by Rystsov: A permutation group G of degree n is primitive if and only if it synchronizes every map of rank n 1 Araújo and I generalised this as follows A block of imprimitivity for G is an equivalence class of a G-invariant equivalence relation G is imprimitive with a block of imprimitivity of size at least k if and only if G fails to synchronize a map with kernel type (k, 1,, 1) Proof If G does not synchronize s, then G, s End(X) for some non-trivial graph X Now the kernel class K of size k of s is collapsed to a point, so must be an independent set; so all edges from K end at points in classes of size 1, and are mapped bijectively by s Thus, any two points in K have the same neighbour set Define a relation on Ω by the rule that v w if v and w have the same neighbour set This is clearly a G-invariant equivalence relation with an equivalence class of size at least k Conversely, if G is imprimitive with blocks of size m k, then it preserves a complete multipartite graph X with blocks of size m; then a map collapsing k points in a block and fixing everything else is an endomorphism of X An improvement Maps of small rank A primitive group of degree n synchronizes every map of rank n 2 Proof The kernel type of such a map is either (3, 1, 1,, 1) or (2, 2, 1,, 1) The first case is dealt with by the preceding theorem In the second case, a careful analysis of the graph shows that the permutation which interchanges the points in the two kernel classes of size 2 and fixes everything else is an automorphism of the graph Now classical results of permutation group theory show that a primitive group containing a double transposition is symmetric or alternating if n 9 The smaller cases can be dealt with directly A primitive group synchronizes every map of rank 2 Proof A graph with an endomorphism of rank 2 is bipartite If G preserves a non-null bipartite graph X, then if X is disconnected, then same connected component is a G-invariant equivalence relation; if X is connected, then same bipartite block is a G-invariant equivalence relation

7 Maps of small rank, 2 By similar arguments, we showed A primitive group synchronizes every non-uniform map of rank 3 or 4 Proof If a monoid M contains a transitive permutation group G, then an element of minimal rank in M is uniform So there is nothing to prove unless the non-uniform map s has rank 4 and the minimal rank is 3, in which case the kernel partition of s splits just one kernel class of a partition of minimal rank In general, if this condition holds, we show that G is imprimitive What s special about primitive graphs? An interesting fact arises from the above proof A primitive graph is a graph admitting an automorphism group which acts primitively on the vertices Apart from symmetry, what is special about primitive graphs? It is easy to see that: Every finite graph occurs as an induced subgraph of some primitive graph The argument about synchronizing gives the following small result in the other direction: Let X be a primitive graph with chromatic number r Then X does not have an induced subgraph isomorphic to K r+1 minus an edge Can any more be said?

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