ABSTRACT
Throughout what follows, a superscript t denotes matrix transpo sition, Greek and Latin subscripts take the values 1, 2 and 1, 2, 3, respectively, and, unless otherwise stipulated, the convention of sum mation over repeated indices is understood. For the sake of simplic ity, we use no special symbols to distinguish between vector-valued and scalar functions, nor between spaces of such functions, the mean ing of the notation being clear from the context.
