Multiplicativity factors for seminorms

AbstractLet A be an algebra and let S be a seminorm on A. In this paper we study multiplicativity factors for S, i.e., constants μ > 0 for which S(xy) ⩽ μS(x) S(y) for all x, y∈A. We begin by investigating these factors in terms of the kernel of S. We then specialize our study to function algebras and to seminorms generated by the sup norm, where we provide a convenient characterization of multiplicativity factors