A Maass Lifting of $\Theta^3$ and Class Numbers of Real and imaginary   Quadratic Fields
  We give an explicit construct of a harmonic weak Maass form $F_{\Theta}$ that is a "lift" of $\Theta^3$, where $\Theta$ is the classical Jacobi theta function. Just as the Fourier coefficients of $\Theta^3$ are related to class numbers of imaginary quadratic fields, the Fourier coefficients of the "holomorphic part" of $F_{\Theta}$ are associated to class numbers of real quadratic fields. 
Robert C. Rhoades and Matthias Waldherr
math.NT