@article {184851,
	title = {Simulating without negation.},
	journal = {Journal of logic and computation},
	volume = {7},
	number = {4},
	year = {1997},
	pages = {501{\textendash}522},
	abstract = {<p>Although negation-free languages are widely used in logic and computer science, relatively little is known about their expressive power. To address this issue we consider kinds of non-symmetric bisimulations called directed simulations, and use these to analyse the expressive power and model theory of negation-free modal and temporal languages. We first use them to obtain preservation, safety and definability results for a simple negation-free modal language. We then obtain analogous results for stronger negation-free languages. Finally, we extend our methods to deal with languages with non-Boolean negation. Keywords: Expressive power, modal logic, negation-free languages.</p>},
	issn = {0955-792X},
	author = {Kurtonina, Natasha and De Rijke, Maarten}
}
