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A base of an action of a group G on a set $\Omega$ is a subset $B \subseteq \Omega$ such that the pointwise stabiliser of $B$ in $G$ is the identity. We prove that if $\Omega$ is the set of partitions of $[ 1, kl ]$ into $l$ subsets of size $k$, then the action of $S_{kl}$ on $\Omega$ has a base of size two if and only if $k \geq 3$ and $l \geq \max \{ k + 3, 8 \}$. This result completes a classification of the primitive base 2 actions of the symmetric groups. During the proof we show that there exists a $k$-regular bipartite graph $\mathcal{G}$ on $2l$ vertices with no non-trivial automorphisms fixing the bipartite blocks if and only if $k \geq 3$ and $l \geq \max \{ k + 3, 8 \}$.

The orientation preserving topological symmetry group of a graph embedded in the 3-sphere is the subgroup of the automorphism group of the graph consisting of those automorphisms which can be induced by an orientation preserving homeomorphism of the ambient space. We characterize all possible orientation preserving topological symmetry groups of embeddings of complete graphs in the 3-sphere.

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PARTITION ACTIONS OF SYMMETRIC GROUPS AND REGULAR BIPARTITE GRAPHS

TOPOLOGICAL SYMMETRY GROUPS OF COMPLETE GRAPHS IN THE 3-SPHERE

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2 results

PARTITION ACTIONS OF SYMMETRIC GROUPS AND REGULAR BIPARTITE GRAPHS

TOPOLOGICAL SYMMETRY GROUPS OF COMPLETE GRAPHS IN THE 3-SPHERE