Pablo Spiga

  1. Generalised quadrangles with a group of automorphisms acting primitively on points and lines.

    Authors: Pablo Spiga, Michael Giudici, John Bamberg, Joy Morris, Gordon F. Royle
    Subjects: Combinatorics
    Abstract

    We show that if G is a group of automorphisms of a thick finite generalised
    quadrangle Q acting primitively on both the points and lines of Q, then G is
    almost simple. Moreover, if G is also flag-transitive then G is of Lie type.

  2. On graph-restrictive permutation groups.

    Authors: Pablo Spiga, Primoz Potocnik, Gabriel Verret
    Subjects: Combinatorics
    Abstract

    Let $\Gamma$ be a connected $G$-vertex-transitive graph, let $v$ be a vertex
    of $\Gamma$ and let $L=G_v^{\Gamma(v)}$ be the permutation group induced by the
    action of the vertex-stabiliser $G_v$ on the neighbourhood $\Gamma(v)$. Then
    $(\Gamma,G)$ is said to be \emph{locally-$L$}. A transitive permutation group
    $L$ is \emph{graph-restrictive} if there exists a constant $c(L)$ such that,
    for every locally-$L$ pair $(\Gamma,G)$ and an arc $(u,v)$ of $\Gamma$, the
    inequality $|G_{uv}|\leq c(L)$ holds.

  3. An Erdos-Ko-Rado theorem for the derangement graph of PGL(2,q) acting on the projective line.

    Authors: Pablo Spiga, Karen Meagher
    Subjects: Combinatorics
    Abstract

    Let G=PGL(2,q) be the projective general linear group acting on the
    projective line P_q. A subset S of G is intersecting if for any pair of
    permutations \pi,\sigma in S, there is a projective point p in P_q such that
    p^\pi=p^\sigma. We prove that if S is intersecting, then the size of S is no
    more than q(q-1). Also, we prove that the only sets S that meet this bound are
    the cosets of the stabilizer of a point of P_q.

  4. An Erdos-Ko-Rado theorem for the derangement graph of PGL(2,q) acting on the projective line.

    Authors: Pablo Spiga, Karen Meagher
    Subjects: Combinatorics
    Abstract

    Let G=PGL(2,q) be the projective general linear group acting on the
    projective line P_q. A subset S of G is intersecting if for any pair of
    permutations \pi,\sigma in S, there is a projective point p in P_q such that
    p^\pi=p^\sigma. We prove that if S is intersecting, then the size of S is no
    more than q(q-1). Also, we prove that the only sets S that meet this bound are
    the cosets of the stabilizer of a point of P_q.

  5. Failure on n-uniqueness: a family of examples.

    Authors: Elisabetta Pastori, Pablo Spiga
    Subjects: Logic
    Abstract

    In this paper, the connections between model theory and the theory of
    infinite permutation groups are used to study the n-existence and the
    n-uniqueness for n-amalgamation problems of stable theories. We show that, for
    any n>1, there exists a stable theory having (k+1)-existence and k-uniqueness,
    for every k<n+1, but that does not have neither (n+2)-existence nor
    (n+1)-uniqueness. In particular, this generalizes the example, for n=2, due to
    E.Hrushovski given in [3].

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