[psacawa]

The accounts in primitve terms obscure it's true meaning.

It's just a analytic function on the moduli space of elliptic curves.

The collection of equivalence classes of elliptic curve (torii of the form C/lattice) has the structure of a complex space (it's not a complex manifold, but rather a complex moduli stack). Modular forms are just analytic functions on it. That's all.

This dumb article doesn't help matter by presenting a brazen lie in the headline. Fifth fundamental operation, my butthurt ass.

[math_dandy]

The title paraphrases a pithy quote of Martin Eichler (I believe), an influential modular forms researcher. It’s not just some pretentious editor trying to be clever.

[enkid]

Sounds like it's a pretentious researcher trying to be clever.

[henrydark]

Well, they're not _just_ that, right?

First, they can be differential forms, not only functions. Second, there's an important note that we don't look only at things over C. For example, specifically in the context of Fermat's Last Theorem, we need Hida's theory of p-adic families of modular forms. Much of the arithmetic of modular forms comes from the modular curves being algebraic and (almost) defined over the integers.

[xyzzyz]

The above definition (analytic function on a moduli space of elliptic curve) actually extends in a natural way. I haven’t known what modular forms were before the parent comment, but I know algebraic geometry, and so it is natural for me to extend above definition for cases you mention.

If modular forms are (global?) sections of the structural sheaf of the moduli space of elliptic curves, the differential forms view will just be the standard construction of sheaf of 1-differentials. Similarly, since elliptic curves are easily defined over arithmetic fields, arithmetic modular forms will just be same thing, but over C_p or something like that.

I actually might be totally off in the above, but I doubt I am: that’s the power of Grothendieck approach, where everything just falls into its natural place in the framework.