In this paper we show that blocking sets of cardinality less than 3(q ϩ 1)/2 (q ϭ p n ) in Desarguesian projective planes intersect every line in 1 modulo p points. It is also shown that the cardinality of a blocking set must lie in a few relatively short intervals. This is similar to previous results of Ré dei, which were proved for a special class of blocking sets. In the particular case q ϭ p 2 , the above result implies that a nontrivial blocking set either contains a Baer-subplane or has size at least 3(q ϩ 1)/2; and this result is sharp. As a by-product, new proofs are given for the Jamison, Brouwer-Schrijver theorem on blocking sets in Desarguesian affine planes, and for Blokhuis' theorem on blocking sets in Desarguesian projective planes.

We show for $k \geq 2$ that if $q\geq 3$ and $n \geq 2k+1$, or $q=2$ and $n \geq 2k+2$, then any intersecting family ${\cal F}$ of $k$-subspaces of an $n$-dimensional vector space over $GF(q)$ with $\bigcap_{F \in {\cal F}} F=0$ has size at most $\left[{n-1\atop k-1}\right]-q^{k(k-1)}\left[{n-k-1\atop k-1}\right]+q^k$. This bound is sharp as is shown by Hilton-Milner type families. As an application of this result, we determine the chromatic number of the corresponding $q$-Kneser graphs.

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In this note we prove that projective planes of order q have defining sets of size o(q 2 ), improving a result of Gray et al. [On the size of the smallest defining set of PG(2, q), Bull. Inst. Combin. Appl. 21 (1997) 91-94].