brosnan@math.ubc.ca

Submission: 2007, Jan 30, revised 2007, Feb 1

We define and study the essential dimension of an algebraic stack. We
compute the essential dimension of the stacks
M
_{ g,n } and M
_{g,n} of
smooth, or stable, n-pointed curves of genus g. We also prove a
general lower bound for the essential dimension of algebraic groups
with a non-trivial center. Using this, we find new exponential lower
bounds for the essential dimension of spin groups and new formulas for
the essential dimension of some finite p-groups. Finally, we apply
the lower bound for spin groups to the theory of the Witt ring of
quadratic forms over a field k.

2000 Mathematics Subject Classification: Primary 14A20; 20G15; 11E04; 14H10

Keywords and Phrases: Essential dimension, algebraic stacks, Witt rings

Full text: dvi.gz 106 k, dvi 235 k, ps.gz 952 k, pdf.gz 424 k, pdf 465 k.

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