Abstract
Let P=PG(2t + 1, q) denote the projective space of order q and of dimension 2t+1≥3. A set ℒ of lines of P is called a blockade if it fulfills the following two conditions.
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1.
Every (t+1)-dimensional subspace of P contains at least one line of ℒ.
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2.
If x is the intersecting point of two lines of ℒ, then every (t+1)-dimensional subspace of P through x contains at least one line of ℓ through x.
The most interesting examples of these blockades are the geometric spreads and the line sets of Baer subspaces of P. In our main result we shall classify the blockades under the additional property that there exists a t-dimensional subspace T of P such that each point of T is incident with at most one line of ℒ. As a corollary we determine the blockades of minimal cardinality.
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Ueberberg, J. Blockades and baer subspaces in finite projective spaces. Geom Dedicata 39, 99–114 (1991). https://doi.org/10.1007/BF00147307
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DOI: https://doi.org/10.1007/BF00147307