The Sandwich Theorem for Sublinear and Superlinear Functionals
  The Hahn-Banach theorem is an extension theorem for linear functionals which preserves certain properties. Specifically, if a linear functional is defined on a subspace of a real vector space which is dominated by a sublinear functional on the entire space, then this functional can be extended to a linear functional on the entire space which is still dominated by a sublinear functional. In this paper, we generalize this result to show that a linear functional defined on a subspace of a real vector space which is dominated by a sublinear functional and also dominates a superlinear functional on the entire space can be extended to a linear functional on the entire space which is also dominated by a sublinear functional and dominates a superlinear functional. 
A.T. Diab, S. I. Nada, D. L. Fearnley
math.FA