TY - JOUR

T1 - Primitive groups, graph endomorphisms and synchronization

AU - Aroújo, J.

AU - Bentz, W.

AU - Cameron, P.J.

AU - Royle, Gordon

AU - Schaefer, A.

PY - 2016

Y1 - 2016

N2 - © 2016 London Mathematical Society.Let O be a set of cardinality n, G be a permutation group on O and f : O ? O be a map that is not a permutation. We say that G synchronizes f if the transformation semigroup contains a constant map, and that G is a synchronizing group if G synchronizes every non-permutation. A synchronizing group is necessarily primitive, but there are primitive groups that are not synchronizing. Every non-synchronizing primitive group fails to synchronize at least one uniform transformation (that is, transformation whose kernel has parts of equal size), and it had previously been conjectured that this was essentially the only way in which a primitive group could fail to be synchronizing, in other words, that a primitive group synchronizes every non-uniform transformation. The first goal of this paper is to prove that this conjecture is false, by exhibiting primitive groups that fail to synchronize specific non-uniform transformations of ranks 5 and 6. As it has previously been shown that primitive groups synchronize every non-uniform transformation of rank at most 4, these examples are of the lowest possible rank. In addition, we produce graphs with primitive automorphism groups that have approximately vn n non-synchronizing ranks, thus refuting another conjecture on the number of non-synchronizing ranks of a primitive group. The second goal of this paper is to extend the spectrum of ranks for which it is known that primitive groups synchronize every non-uniform transformation of that rank. It has previously been shown that a primitive group of degree n synchronizes every non-uniform transformation of rank n - 1 and n - 2, and here this is extended to n - 3 and n - 4. In the process, we will obtain a purely graph-theoretical result showing that, with limited exceptions, in a vertex-primitive graph the union of neighbourhoods of a set of vertices A is bounded below by a function that is asymptotically v|A|. Determining the exact spectrum of ranks for which there exist non-uniform transfor

AB - © 2016 London Mathematical Society.Let O be a set of cardinality n, G be a permutation group on O and f : O ? O be a map that is not a permutation. We say that G synchronizes f if the transformation semigroup contains a constant map, and that G is a synchronizing group if G synchronizes every non-permutation. A synchronizing group is necessarily primitive, but there are primitive groups that are not synchronizing. Every non-synchronizing primitive group fails to synchronize at least one uniform transformation (that is, transformation whose kernel has parts of equal size), and it had previously been conjectured that this was essentially the only way in which a primitive group could fail to be synchronizing, in other words, that a primitive group synchronizes every non-uniform transformation. The first goal of this paper is to prove that this conjecture is false, by exhibiting primitive groups that fail to synchronize specific non-uniform transformations of ranks 5 and 6. As it has previously been shown that primitive groups synchronize every non-uniform transformation of rank at most 4, these examples are of the lowest possible rank. In addition, we produce graphs with primitive automorphism groups that have approximately vn n non-synchronizing ranks, thus refuting another conjecture on the number of non-synchronizing ranks of a primitive group. The second goal of this paper is to extend the spectrum of ranks for which it is known that primitive groups synchronize every non-uniform transformation of that rank. It has previously been shown that a primitive group of degree n synchronizes every non-uniform transformation of rank n - 1 and n - 2, and here this is extended to n - 3 and n - 4. In the process, we will obtain a purely graph-theoretical result showing that, with limited exceptions, in a vertex-primitive graph the union of neighbourhoods of a set of vertices A is bounded below by a function that is asymptotically v|A|. Determining the exact spectrum of ranks for which there exist non-uniform transfor

U2 - 10.1112/plms/pdw040

DO - 10.1112/plms/pdw040

M3 - Article

VL - 113

SP - 829

EP - 867

JO - Proceedings of London Mathematical Society

JF - Proceedings of London Mathematical Society

SN - 0024-6115

IS - 6

ER -