Sigmund, K. <https://pure.iiasa.ac.at/view/iiasa/282.html>  (1995).  Hans Hahn and the foundational debate.    In:  The Foundational Debate Complexity and Constructivity in Mathematics and Physics. Eds. DePauli-Schimanovich, W., Köhler, E., & Stadler, F., pp. 235-245 Springer.  ISBN 978-94-017-3327-4