Résumé  Minimization of Buchi automata is an intriguing topic in automata theory,
both for a theoretical understanding of automata over infinite words, and for practical applications.
Ideally, for a given language, one would like to find an automaton recognizing it with the least number of states.
Since exact minimization is computationally hard (e.g., PSPACEcomplete),
we concentrate on quotienting, which is a statespace reduction technique which works by "glueing together" certain states.
Which states can be merged is dictated by suitable preorders:
In this talk, we study fixedword simulations, which are simulationlike preorders sound for quotienting.
We show that fixedword simulations are coarser than previously studied simulationlike preorders,
by characterizing it with a natural (but nontrivial) ranking argument.
Our ranking construction is related to the socalled KupfermanVardi construction for complementing Buechi automata.
