The exact spread of M12 is 9

Abstract

Let G be a group. We say that G has spread r if for any set of distinct non-trivial elements {x1,...,xr}\subset G there exists an element y\in G with the property that <xi, y> = G for every 1 0<i<r+1. The group G has exact spread r if it has spread r but not r + 1. The case where G is a finite simple group is particularly interesting since it is known that in this case the spread is at least 2. The precise value of the exact spread of a simple group is known in very few cases. Here we determine the precise value of the exact spread in the smallest sporadic group for which this is still unknown, the Mathieu group M12.