P.2. A bird’s eye view of hyperbolic equations xix
My research was partially supported by a sequence of National Science
Foundation grants; NSF-DMS-9203413, 9803296, 0104096, and 0405899.
The early stages of the research on geometric optics were supported by
the Oﬃce of Naval Research under grant OD-G-N0014-92-J-1245. My visits
to Paris have been supported by the CNRS, the Fondation Math´ematiques
de Paris, and the Institut Henri Poincar´ e. My visits to Pisa have been sup-
ported by the the Universit` a di Pisa, INDAM, GNAMPA, and the Centro
di Ricerca Matematica Ennio De Giorgi. Invitations as visiting professor
at many departments (and notably l’Universit´ e de Paris Nord, l’Ecole
male Sup´ erieure, and l’Ecole
Polyt´ echnique for multiple invitations) have
been crucially important in my research. I sincerely thank all these organi-
zations and also the individuals responsible.