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Nov 14, 2015 at 18:25 comment added Will Sawin @მამუკაჯიბლაძე If you want to understand everything about how the local theory relates to the global theory, I think yes. You can express everything with classical modular forms, but some things will be less elegant.
Nov 13, 2015 at 8:53 comment added მამუკა ჯიბლაძე So do you imply that ultimately to understand all this one has to translate modular forms into the language of automorphic representations?
Nov 12, 2015 at 13:54 comment added Will Sawin @მამუკაჯიბლაძე There is. From a modular form and a prime $p$ you obtain an irreducible representation of $GL_2(\mathbb Q_p)$. This is from viewing the modular form as a differential form on $\lim_{n \to \infty} X(p^n)$, which has an action of $GL_2(\mathbb Q_p)$, and taking the representation it generates, which turns out to be irreducible. This is the "reduction mod $p$".
Nov 12, 2015 at 6:57 comment added მამუკა ჯიბლაძე As for your last remark - this is in fact a good example to argue that there are never any pure coincidences. After all, your equality of Lie groups can be explained by $\mathbb C\mathbf P^1=S^2$. You can call also this a coincidence but...
Nov 12, 2015 at 6:54 comment added მამუკა ჯიბლაძე This is a very good answer, thank you! The key seems to be in the third paragraph, may I ask you to just slightly expand it? I mean, is there any sense in the combination of words "reduction of a modular form at a prime"?
Nov 12, 2015 at 4:49 history answered Will Sawin CC BY-SA 3.0