A Maass form (of weight 0) of level 1 is a smooth, square-integrable, automorphic eigenform of the Laplace-Beltrami operator $\Delta$ that is invariant under $SL(2,\mathbb{Z})$. The Laplace eigenvalue of a Maass form has the form $\lambda = 1/4 + R^2$, where $R > 0$ is called the spectral parameter.
