The paper focuses on 5' to 3' Watson-Crick (WK) automata, which work on a DNA-like tape and have two reading heads moving in opposite directions. It considers the case where the input represents circular DNA molecules, known as necklaces.
The key insights are:
Weak acceptance mode: A necklace is accepted if any of its conjugates (cyclic shifts) is accepted by the automaton. These languages are shown to be the cyclic closure of linear context-free languages.
Strong acceptance mode: A necklace is accepted only if all of its conjugates are accepted by the automaton. These languages exhibit a "locally testable" property, where the acceptance depends on the presence of certain patterns in the necklace.
The paper presents a series of hierarchy results comparing the expressive power of various restricted variants of 5' to 3' WK automata, such as stateless, all-final, simple, and 1-limited, under both weak and strong acceptance modes.
It is shown that the class of necklace languages weakly accepted by 5' to 3' WK automata is the cyclic closure of the linear context-free languages, while the class of necklace languages strongly accepted by these automata has a "locally testable" property.
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