On [formula omitted]-neighborly reorientations of oriented matroids.

We study the existence and the number of k -neighborly reorientations of an oriented matroid. This leads to k -variants of McMullen's problem and Roudneff's conjecture, the case k = 1 being the original statements. Adding to results of Larman and García-Colín, we provide new bounds on k -McMullen's problem and prove the conjecture for several ranks and k by computer. Further, we show that k -Roudneff's conjecture for fixed rank and k reduces to a finite case analysis. As a consequence we prove the conjecture for odd rank r and k = r − 1 2 as well as for rank 6 and k = 2 with the aid of the computer. [ABSTRACT FROM AUTHOR]