Volume 458, 2008
Percy Deift at integer times
To Percy, with deep affection, admiration and gratitude.
This is the first of two lectures dedicated to a description of some
of Percy's mathematical achievements.
During the meeting arranged to honor Percy's 60th birthday, two lectures were
dedicated to a description of (part of) his work. This text concentrates on Percy's
contributions on scattering theory on the line and integrable systems up to the
early nineties. On the other hand, this is not a short history of either subject.
The seventies and eighties were very active times, and I had to limit myself to few
examples, emphasizing aspects which are relevant to Percy's current activities.
In retrospect, it is remarkable how these areas are intertwined. Scattering
theory provide action-angle variables to infinite dimensional evolutions, Riemann-
Hilbert problems come up in the solution of differential equations by factorization
formulas. Schrodinger operators and tridiagonal matrices follow parallel concep-
tual tracks and band matrices indicate that infinitely many integrals for certain
non-linear PDE's are not enough to assert complete integrability. Conserved quan-
tities arise not only from physics, but from numerical analysis: whatever is being
approximated by an iterative algorithm may not change along the computation.
As a student of Percy, I frequently had the impression that things moved faster
that I could follow. The feeling did not change.
2. Direct and inverse scattering
Percy's contributions to the scattering theory of operators on the line con-
centrate on two texts. Inverse Scattering on the Line (ISL,
Trubowitz, presents a full account of the direct and inverse scattering theory for
the Schrodinger equation on the line with real potential,
2000 Mathematics Subject Classification:
Primary 34125, 35Q15, 70H06; Secondary 35Q58
Key words and phrases.
Scattering theory, integrable systems.
The author is thankful for the many suggestions presented by a referee. This work is sup-
ported by CNPq and Faperj.