Guido Gherardi Alberto Marcone How Incomputable is the Separable Hahn-Banach Theorem? 85-102 2008 221 Electr. Notes Theor. Comput. Sci. http://dx.doi.org/10.1016/j.entcs.2008.12.009 db/journals/entcs/entcs221.html#GherardiM08