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Abstract In this paper, we show that $$\lambda (z_1) -\lambda (z_2)$$, $$\lambda (z_1)$$, and $$1-\lambda (z_1)$$ are all Borcherds products on $$X(2) \times X(2)$$. We then use the big CM value formula of Bruinier, Kudla, and Yang to give explicit factorization formulas for the norms of $$\lambda (\frac{d+\sqrt d}2)$$, $$1-\lambda (\frac{d+\sqrt d}2)$$, and $$\lambda (\frac{d_1+\sqrt{d_1}}2) -\lambda (\frac{d_2+\sqrt{d_2}}2)$$, with the latter under the condition $$(d_1, d_2)=1$$. Finally, we use these results to show that $$\lambda (\frac{d+\sqrt d}2)$$ is always an algebraic integer and can be easily used to construct units in the ray class field of $${\mathbb{Q}}(\sqrt{d})$$ of modulus $$2$$. In the process, we also give explicit formulas for a whole family of local Whittaker functions, which are of independent interest.
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Schemes Supported on the Singular Locus of a Hyperplane Arrangement in ℙ n
Abstract A hyperplane arrangement in $$\mathbb P^n$$ is free if $R/J$ is Cohen–Macaulay (CM), where $$R = k[x_0,\dots ,x_n]$$ and $$J$$ is the Jacobian ideal. We study the CM-ness of two related unmixed ideals: $$ J^{un}$$, the intersection of height two primary components, and $$\sqrt{J}$$, the radical. Under a mild hypothesis, we show these ideals are CM. Suppose the hypothesis fails. For equidimensional curves in $$\mathbb P^3$$, the Hartshorne–Rao module measures the failure of CM-ness and determines the even liaison class of the curve. We show that for any positive integer $$r$$, there is an arrangement for which $$R/J^{un}$$ (resp. $$R/\sqrt{J}$$) fails to be CM in only one degree, and this failure is by $$r$$. We draw consequences for the even liaison class of $$J^{un}$$ or $$\sqrt{J}$$.
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- Award ID(s):
- 2048906
- PAR ID:
- 10361502
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- International Mathematics Research Notices
- Volume:
- 2022
- Issue:
- 1
- ISSN:
- 1073-7928
- Page Range / eLocation ID:
- p. 140-170
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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