For what I have heard, Maass forms of (Laplacian) eigenvalue $1/4$ on modular surfaces are somewhat special. But I don't know where to look for explicit examples. (In fact, one form came here on MO Does anyone want a pretty Maass form?, but I am not sure I can figure out what it is from the description.) So, can anyone help me with references?
Take the 2minute tour
×
MathOverflow is a question and answer site for professional mathematicians. It's 100% free, no registration required.
One place you can look is the paper by Booker and Strombergsson which appeared in Crelle. Their aim is to verify the Selberg eigenvalue conjecture in a number of cases, and when there are Maass forms of eigenvalue $1/4$ these must be accounted for by finding corresponding Galois representations. See Section 5 of the paper for some explicit examples; of course there is a lot of other work on this, but the paper will give you more references. 

