Blocking sets of external, tangent and secant lines to a quadratic cone in [formula omitted].

Consider a quadratic cone K in the 3-dimensional projective space PG (3 , q) over a finite field of order q , where q is a prime power. Let E (respectively, T , S) denote the set of all lines of PG (3 , q) that are external (respectively, tangent, secant) with respect to K. We characterize the minimum size blocking sets in PG (3 , q) with respect to the line set A , where A is one of E , T , S , E ∪ T , E ∪ S and T ∪ S. [ABSTRACT FROM AUTHOR]