lectures—I wa s less inhibited about using ideas from differentia l equations . The
books [Au4, GT], and the appendix to [Be2] may help fill this gap.
I will be pleased if these lectures make the problems and ideas discussed more
accessible to nonexperts.
It is a pleasure to thank Craig Evans and Neil Trudinger for generously sharing
their idea s on th e boundary regularit y in the last section , and t o Luis Caffarell i
for the use of his contributions in that same section. I also benefited fro m Denni s
DeTurck's many suggestions for improvements.
Many people helped make this conference enjoyable and rewarding to me. The
main organizationa l detail s wer e superbl y execute d b y Edwar d Miller , Rober t
Sibner, and especially Lesley Sibner. I am grateful to them.
Finally, I would like to thank Frank Warner for getting me interested in these
problems fifteen year s ago.