On Non-looping Term Rewriting

Proving non-termination is important for instance if one wants to decide termination for given TRSs. Although the usual method is to find looping reduction sequences, there are non-looping infinite reduction sequences. We find some new interesting non-looping examples and propose new definitions of inner-looping sequence and normal sequence to classify them. We also show the undecidability of the existence of inner-looping sequence.