Abstract
*Partially supported by the NSF and the Sloan Foundation.
†Partially supported by the Missouri Research Council.
ACKNOWLEDGMENTS
We would like to acknowledge useful conversations with I. Madsen, which inspired this paper, and with L. Avramov, which simplified the arguments in the first section. We would also like to thank the referee for suggesting the alternative proof of Lemma 3.1.
Notes
*Partially supported by the NSF and the Sloan Foundation.
†Partially supported by the Missouri Research Council.