Dear readers,

I know I promised a post on modular curves, but I had to devote more time to my end of semester project. Since it’s strongly related to the topic of modular curves and I present on it tomorrow, I decided to make this post in the seminar talks category.

First we say something about modular forms and modular curves, but maybe with a bit of added generality. Consider a discrete subgroup of . By the action of on , acts on the upper half plane since it must preserve and not switch the upper and lower half-planes.

A modular form for of weight is then a holomorphic function on the upper-half plane which satisfies two properties:

- If then .
- is holomorphic at limiting points called cusps.

We pay little attention to the second condition in our case of modular forms on quaternion algebras. In general, a modular form will have a wealth of information found in the limiting points, which give a Fourier expansion for the modular form. In our case, we will be dealing with modular forms which live on compact manifolds.

One might ask oneself here, what do modular forms have to do with manifolds? The answer is that we can look at the upper half plane modulo the action of the discrete group and by adding the cusps to this quotient manifold, we get a compact complex 1-manifold(or if you prefer, a compact Riemann Surface). The modular forms of weight which vanish at all cusps play a very important role as if is such a “cusp form” then is a holomorphic differential on (or if you prefer, a global section of the sheaf of holomorphic differentials). Indeed the map described gives an isomorphism of complex vector spaces.

So now what are quaternion algebras and what can they do for us? Well there’s the correct definition which I will give in my talk tomorrow and I will surely revisit here at some point when I want to talk about the Brauer group and the “good enough” definition which is correct as long as the characteristic of your field is not 2, which I give now.

**Definition:**A quaternion algebra over a field of characteristic not 2 is a 4-dimensional algebra such that .

**Example:** The only quaternion algebra you are guaranteed to have over any field is the 2×2 matrices .

Defining a quaternion algebra in this manner gives a natural involution where if then . Thus because and are both invariant under the involution, and thus must belong to . We denote to respectively be the reduced trace and reduced norm of .

One natural thing to consider is how a quaternion algebra changes when we extend the field of definition. For instance, is not isomorphic to but is, because only matter up to squares and both 2 and 3 are squares in .

**Definition:** A rational quaternion algebra is said to be indefinite if when we extend the base field to it becomes isomorphic to and definite otherwise. This choice of terminology comes from the theory of quadratic forms, indeed is definite/indefinite if the norm form is a definite/indefinite quadratic form.

Indefinite quaternion algebras over the rational numbers correspond to modular forms by way of orders.

**Definition:**A -order in a rational quaternion algebra is a -algebra of elements with integral reduced trace and reduced norm inside of such that . We always have at least one, .

If is an indefinite quaternion algebra, any order of can be realized as a discrete subalgebra of the real split quaternions, indeed and the invertible elements of reduced norm(one can check that this is now identified with the determinant) 1 embed as a discrete subgroup of so we can consider modular forms for .

Moreover, classical modular forms for congruence subgroups of arise in exactly this way because is a -order in . To see this directly, consider the congruence subgroup .

We can realize this as the invertible elements of reduced norm 1 in the order

Likewise can be realized as the invertible elements of reduced norm 1 in .

One very special thing we can do with is to create a modularity map from the modular curve to an elliptic curve using a special sort of cusp form of weight 2, called a newform. For details on this construction, the reader is heartily recommended to check out A First Course in Modular Forms.It is the infamous Taniyama-Shimura Conjecture (now proved due to Breuil-Conrad-Diamond-Taylor-Wiles) that every elliptic curve may be realized in this way.

Well it turns out that using a result of Zhang from 2001, you can do the same with any newform of weight 2 on any quaternionic modular curve.

I have left out a number of things I plan to talk about, but this is a good start. After the talk I will post the notes I’ve written up for it.

April 22, 2009 at 6:26 pm

Nice post, and very interesting. I look forward to read you notes!

April 26, 2009 at 2:40 pm

Finally, the notes. The talk went well, although I had to blaze through just about everything that isn’t on the first 3 pages

http://math.uga.edu/~jstankew/quatalg.pdf

June 24, 2009 at 12:02 am

Minor detail (near the top): only the invertible 2×2 matrices act on P^1.

July 21, 2010 at 11:14 am

nice job. please write more about these quaternion algebras over the rationals. be well.