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The Siegel Modular Variety of Degree Two and Level Four/Cohomology of the Siegel Modular Group of Degree Two and Level Four
Ronnie Lee, Yale University, New Haven, CT, and Steven H. Weintraub and J. William Hoffman, Louisiana State University, Baton Rouge, LA
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Memoirs of the American Mathematical Society
1998; 75 pp; softcover
Volume: 133
ISBN-10: 0-8218-0620-3
ISBN-13: 978-0-8218-0620-3
List Price: US$44 Individual Members: US$26.40
Institutional Members: US\$35.20
Order Code: MEMO/133/631

The Siegel Modular Variety of Degree Two and Level Four, by Ronnie Lee and Steven H. Weintraub

Let $$\mathbf M_n$$ denote the quotient of the degree two Siegel space by the principal congruence subgroup of level $$n$$ of $$Sp_4(\mathbb Z)$$. $$\mathbf M_n$$ is the moduli space of principally polarized abelian surfaces with a level $$n$$ structure and has a compactification $$\mathbf M^*_n$$ first constructed by Igusa. $$\mathbf M^*_n$$ is an almost non-singular (non-singular for $$n > 1$$) complex three-dimensional projective variety (of general type, for $$n > 3$$).

The authors analyze the Hodge structure of $$\mathbf M^*_4$$, completely determining the Hodge numbers $$h^{p,q} = \dim H^{p,q}(\mathbf M^*_4)$$. Doing so relies on the understanding of $$\mathbf M^*_2$$ and exploitation of the regular branched covering $$\mathbf M^*_4 \rightarrow \mathbf M^*_2$$.

Cohomology of the Siegel Modular Group of Degree Two and Level Four, by J. William Hoffman and Steven H. Weintraub

The authors compute the cohomology of the principal congruence subgroup $$\Gamma_2(4) \subset S{_p4}(\mathbb Z)$$ consisting of matrices $$\gamma \equiv \mathbf 1$$ mod 4. This is done by computing the cohomology of the moduli space $$\mathbf M_4$$. The mixed Hodge structure on this cohomolgy is determined, as well as the intersection cohomology of the Satake compactification of $$\mathbf M_4$$.

Graduate students and research mathematicians working in algebraic geometry.

The Siegel Modular Variety of Degree Two and Level Four
• Introduction
• Algebraic background
• Geometric background
• Taking stock
• Type III A
• Type II A
• Type II B
• Type IV C
• Summing up
• Appendix. An exact sequence in homology
• References
Cohomology of the Siegel Modular Group of Degree Two and Level Four
• Introduction
• The building
• Cycles
• The main theorems
• References