On Approximating Real-World Halting Problems
S. Köhler, C. Schindelhauer, M. Ziegler, in: Fundamentals of Computation Theory, Berlin, Heidelberg, 2005.

An Explicit Solution to Post’s Problem over the Reals
K. Meer, M. Ziegler, in: Fundamentals of Computation Theory, Berlin, Heidelberg, 2005.