@inproceedings{FORMATS '07: Undecidability of Universality for Timed Automata with Minimal Resources,
  title = "Undecidability of Universality for Timed Automata with Minimal Resources",
  author = "Sara Adams and Joel Ouaknine and James Worrell",
  year = "2007",
  booktitle = "Formal Modeling and Analysis of Timed Systems 2007",
  isbn = "978-3-540-75453-4",
  pages = "25-37",
  publisher = "Springer",
  series = "Lecture Notes in Computer Science",
  url = "http://www.sara-adams.de/files/formats07.pdf",
  volume = "4763",
}