We consider stateless counter machines which mix the features of one-head counter machines and a special type of two-head Watson-Crick automata (WK-automata). Our biologically motivated machines have heads that read the input starting from the two extremes. The reading process is finished when there are no more symbols between the heads. Depending on whether the heads are required to advance at each move, we distinguish between realtime and non-realtime machines. If every counter makes at most

more »

... alternations between nondecreasing and decreasing modes in every computation, then the machine is k-reversal. In this paper we concentrate on the properties of nondeterministic stateless WK-automata with counters. Results about deterministic versions can be found in (Egecioglu et al.: Stateless multicounter 5 → 3 Watson-Crick Automata, BIC-TA 2010).