Mathematics subject classification: Primary 11F33 · Secondary 11F55Abstract We show that certain p-adic Eisenstein series for quaternionic modular groups of degree 2 become "real" modular forms of level p in almost all cases. To prove this, we introduce a U (p) type operator. We also show that there exists a p-adic Eisenstein series of the above type that has transcendental coefficients. Former examples of p-adic Eisenstein series for Siegel and Hermitian modular groups are both rational (i.e., algebraic).
Preliminaries
Notation and definitionsLet H be Hamiltonian quaternions and O the Hurwitz order (cf. [6]). The half-space of quaternions of degree n is defined as H(n; H) := { Z = X + iY | X, Y ∈ Her n (H), Y > 0 }. Let J n := O n 1 n −1 n O n . Then, the group of symplectic similitudes M ∈ M (2n, H) | t M J n M = qJ n for some positive q ∈ R acts on H(n; H) by Z −→ M Z = (AZ + B)(CZ + D) −1 , M = A B C D .