Lecture given at the Swedish Academy of Sciences on
October 24 2001, on the occasion of Elliott Lieb being awarded the
2001 Rolf Schock Prize. (A somewhat different version of this talk was
given at the Centennial celebration of the birth of Enrico Fermi in
Rome September 29, 2001)
Lecture given at the
Conference in Partial Differential Operators and Spectral Theory held
in honor of M.S. Birman at The Royal Institute of Technology,
Stockholm, January 3-7 1998.