|Original language||English (US)|
|Number of pages||21|
|Journal||Proceedings of the London Mathematical Society|
|State||Published - Nov 2000|
Bibliographical noteFunding Information:
The research of the first author was supported in part by NSF Grant DMS 98-03154. That of the second author was supported by a TALENT-stipendium from Netherlands Organization for Scientific Research (NWO). 1991 Mathematics Subject Classification: 13A50, 16–XX, 22E70, 35A30, 35Q53, 58F07.