| Résume||L'exposé sera diffusé (à préciser, demander le mot de passe à Oliver B. ou D. ou Frederic H.). L'orateur sera à distance.
Abstract: I will explain the question of uniqueness for centers of blow ups and blow downs of birational maps, state what is currently known and give two applications. The first is to the structure of Cremona groups, namely their nongeneration by involutions in dimension >= 3. The second application is for the Grothendieck ring of varieties, of dimension <= 2, over perfect fields. Based on two papers in preparation with H.-Y. Lin, and with H.-Y. Lin and S. Zimmermann.|