Step 1 and Step 2 are now proved. Hence, G(Hk
n )
∼=
G(Sk)
∼=
Sk , as desired.
ACKNOWLEDGMENTS. I would like to thank Professors Jason Behrstock and Elisenda Grigsby, the graduate
student advisor, Harold Sultan, and the participants in the Columbia University REU program, especially
Michael Rand for offering the figures for the proof as well as the idea for the proof of Lemma 4. I would like
to thank the Columbia University math department, Professors Robert Friedman and Peter Ozsv´ath, and the
National Science Foundation (DMS-0739392) for organizing and supporting the program. I am grateful to the
referees for their comments.