We prove that the coefficients of certain weight -1/2 harmonic Maass forms
are traces of singular moduli for weak Maass forms. To prove this theorem, we
construct a theta lift from spaces of weight -2 harmonic weak Maass forms to
spaces of weight -1/2 vector-valued harmonic weak Maass forms on Mp_2(Z), a
result which is of independent interest. We then prove a general theorem which
guarantees (with bounded denominator) when such Maass singular moduli are
algebraic.
We give a formula for the values of automorphic Green functions on the
special rational 0-cycles (big CM points) attached to certain maximal tori in
the Shimura varieties associated to rational quadratic spaces of signature
(2d,2). Our approach depends on the fact that the Green functions in question
are constructed as regularized theta lifts of harmonic weak Mass forms, and it
involves the Siegel-Weil formula and the central derivatives of incoherent
Eisenstein series for totally real fields.
We define a regularized theta lift for orthogonal groups over totally real
fields generalizing work of Borcherds. The lift takes harmonic `Whittaker
forms' to automorphic Green functions and weakly holomorphic Whittaker forms to
meromorphic modular forms on orthogonal groups with zeros and poles supported
on special divisors.