C. M. Petty has conjectured the minimum value for a certain affine-inv
ariant functional defined on the class of convex bodies. We give sharp
bounds for this functional on a certain subclass of convex bodies, an
d we give a counterexample to an upper bound proposed by R. Schneider
for the class of centrally symmetric convex bodies. We conjecture that
the simplex provides the maximum on the class of all convex bodies, w
hile the largest centrally symmetric subset of a simplex gives a sharp
upper bound on the class of all centrally symmetric convex bodies.