Post's Correspondence Problem: All You Can Count
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(Mirko Rahn)


The Post correspondence problem is to determine whether the equality
set of two morphisms contains more than the empty word. It is
undecidable in general as shown by Post in 1946.

Sometimes it is possible to decide or at least simplify instances by
counting arguments. We present such arguments, show how to view them
from different perspectives and generalize it to a comprehensive
framework. Finally we push the framework to its limits by combining it
with other approaches. The resulting technique is quite powerful as
empirical data shows.