Vasco Brattka Matthias Schröder Klaus Weihrauch 2002 66 Electr. Notes Theor. Comput. Sci. 1 http://www.elsevier.com/gej-ng/31/29/23/120/49/show/Products/notes/index.htt#001 db/journals/entcs/entcs66.html#BrattkaSW02