Egon Börger. Spektralproblem and completeness of logical decision problems. In Egon Börger, Gisbert Hasenjaeger, Dieter Rödding, editors, Logic and Machines: Decision Problems and Complexity, Proceedings of the Symposium Rekursive Kombinatorik held from May 23-28, 1983 at the Institut für Mathematische Logik und Grundlagenforschung der Universität Münster/Westfahlen. Volume 171 of Lecture Notes in Computer Science, pages 333-356, Springer, 1983.