Characteristic Formulae for Timed Automata Luca Aceto, Anna Ingólfsdóttir, Mikkel Lykke Pedersen and Jan Poulsen RAIRO-Theor. Inf. Appl., 34 6 (2000) 565-584 Published online: 15 April 2002 DOI: 10.1051/ita:2000131