It is well known and mentioned/proved in most introductory texts that the only automorphic forms on lower general groups are (constant functions and other details aside):
$GL(1)$: Hecke characters
$GL(2)$: modular forms and Maass forms
Then they usually consider $GL(n)$ directly.
My question is:
- What are the automorphic forms in $GL(3)$?
I am aware of this other question on MO: Automorphic forms on GL(3), which deals with a related issue. In particular, a partial answer seems to be given:
$GL(3)$: Maass forms and Gelbart-Jacquet lifts.