1. Department of Mathematics, Duke University, Durham, North Carolina, U.S.A.
2. Department of Mathematics, Stony Brook University, Stony Brook, New York, N.Y., U.S.A.; and Department of Mathematics, Duke University, Durham, North Carolina, U.S.A.
3. Department of Mathematics, Stony Brook University, Stony Brook, New York, N.Y., U.S.A.