Let S be any nontrivial blocking set in a projective plane of order q,
such that S not containing any line. 
Let A be a set of points in the same projective plane of order q,
raging over all these S.


Is it true that if A$\cap$S != $\phi$ then |A| >= q+1
and equality exists only if A is a line?



Blocking set in a projective plane.
Let $\ell\_1,\ell\_2,\ell\_3$ be three lines in the plane that do not all contain the same point. The *triangle* formed by $\ell\_1,\ell\_2,\ell\_3$ is the set obtained from $\ell\_1 \cup \ell\_2 \cup \ell\_3$ by removing the three pairwise intersections of the lines. Clearly, any *triangle* is a blocking set: every line in the plane meets each line $\ell\_1,\ell\_2,\ell\_3$, and cannot contain all the three points that we are removing from the *triangle*. *Triangles* are the only blocking sets we consider.


We say that $A$ has property (`*`) if the intersection of $A$ with every *triangle* is not empty. Note that if a subset of the plane intersects non-trivially every blocking set, then it has property (`*`). We prove the (*a priori*) stronger statement that a set with property (`*`) and at most $q+1$ elements consists of all the points contained in a line.


**Lemma 1.**
*Let $A$ be a set of at least three points in the plane such that any line intersects $A$ in at most two points; then $A$ cannot have property (`*`).*


*Proof.*
Let $A' \subset A$ be a subset with three elements, and note that the points in $A'$ are not collinear. Let $\ell\_1,\ell\_2,\ell\_3$ be the three lines each containing two of the points of $A'$. By assumption $\ell\_1 \cap A , \ell\_2 \cap A , \ell\_3 \cap A \subset A'$; thus, the *triangle* formed by $\ell\_1 , \ell\_2 , \ell\_3$ is a blocking set disjoint from $A$. $\square$


**Lemma 2.**
*Let $A$ be a set of at most $q+1$ points in the plane that are not collinear and let $\ell$ be a line in the plane such that $|A \cap \ell| \geq 3$; then $A$ cannot have property (`*`).*


*Proof.*
Let $a$ be a point of $\ell \setminus A$: such a point exists, since the points of $A$ are not collinear and $\ell$ contains $q+1$ points. There are $q$ lines different from $\ell$ through $a$ and at most $q-2$ points in $A \setminus \ell$; thus there are two distinct lines $\ell\_1 , \ell\_2$ through $a$ disjoint from $A$. Let $b'$ be any point of $A$ not contained in $\ell$ and let $b$ be any point not contained in $A$ on a line $\ell'$ joining $b'$ to a point of $A$ contained in $\ell$; there are $q$ lines through $b$ different from $\ell'$ and only $q-1$ points in $A \setminus \ell'$; we deduce that there is a line $\ell\_3$ through $b$ disjoint from $A$. Thus the *triangle* formed by $\ell\_1,\ell\_2,\ell\_3$ is a blocking set disjoint from $A$. $\square$


**Corollary.**
*Let $A$ be a set of at most $q+1$ points in the plane having property (`*`); then $A$ consists of all the points contained in a line.*


*Proof.*
By Lemma 1 we know that there must be a line containing three points of $A$. If the points of $A$ were not collinear, then Lemma 2 would imply that $A$ does not have property (`*`). Thus the points of $A$ are collinear. To conclude it suffices to show that if $A$ misses a point of the line containing $A$, then $A$ does not have property (`*`). This is easy: let $a$ be a point of $A$ and let $\ell\_1,\ell\_2$ be distinct lines through $a$; let also $b$ be a point on the line containing $A$ not in $A$, and let $\ell\_3$ be any line through $b$ not containing $A$. The *triangle* formed by $\ell\_1,\ell\_2,\ell\_3$ is a blocking set disjoint from $A$. $\square$


