1. In the sense of being distinct from the space S.
2. Uniqueness is a consequence of the convexity of M(CK), if CK is chosen to be convex.
3. P. C. Sabatier, “Basic Concepts of Inverse Problems,” inTomography and Inverse Problems, edited by P. C. Sabatier (Hilger, Bristol, 1987).
4. M. Ciulli, S. Ciulli, and T. D. Spearman, “Ball Stabilisation of Inverse Problems,” inInverse Methods in Action, edited by P. C. Sabatier (Springer, Heidelberg, 1989).