So now we begin one of the great examples in algebraic geometry: algebraic groups. These are exceptionally nice, and we’ll talk about a couple of more general concepts before applying them to this case.
Before we can have any hope of talking about algebraic groups, we need to talk about products of varieties. We’re going to talk about our varieties without any reference to the affine or projective space that it is embedded in, and so we actually need to look at the Zariski topology on each of them.
Now, we can’t just say that the closed sets are the products of closed sets. Why not? Well, look at . The products of closed sets are just finite collections of points and vertical and horizontal lines. In particular, the parabola isn’t a closed set! This is bad, because we would LIKE to have . The point set is right, we just need a good criteria for determining the topology.
So we’ll define the product of two varieties to be the variety (and pair of morphisms such that given any other variety with morphisms and , there is a unique morphism such that and .
So what is this really saying? Well, we call the maps the projection maps, because the product has point set , and and . So the universal property, that is, the map is really to make sure that the topology works out.
We should note that though now we have , we DON’T have . This is VERY false. Though we do get that the product of affine varieties is affine and the product of projective varieties is projective. (This last bit requires a trick called the Segre Embedding which we won’t talk about).
So now on to algebraic groups. An algebraic group is a variety (affine, projective, quasi-projective…whatever) along with morphisms , and . (the last one is just choosing a point of , we’ll denote this point by ) These morphisms must satisfy a bunch of relations, specifically , and . These are just the axioms of a group, and so an algebraic group is just a variety which is also a group via morphisms. (This definition can be generalized to a group object in a category, and we will need this generality later, but for now, we’ll focus on varieties.)
So now, let’s mention a couple of examples. First, any group of matrices is an algebraic group. We can just look at to check this. Now, an invertible matrix is determined by the inequality , and so is by definition a quasi-affine variety in . However, we can do better. If we stick on another variable, so we’re working on as the coordinate ring of our affine space, we can then look at solutions to , where is the determinant polynomial in terms of the . This condition requires that the determinant be nonzero, and has no other requirements! Thus, is an affine variety in .
Now we need to check that inversion and multiplication are morphisms. From last time, we know that morphisms of affine varieties are the same as ring homomorphisms of the coordinate rings. So now if we take two matrices, the multiplication can be given by a bunch of polynomials in the coordinates, that is, , and so this is a morphism. Inversion is similar, though we need to divide by the determinant, but that’s ok, because it’s nonzero on our varieties. Thus, matrix groups are algebraic groups.
Anyone who knows a bit of the theory of Lie groups will recognize these as the standard examples. We’ll make this a bit more careful next, and that will be it for the day.
We say that a variety is nonsingular at a point if . So that’s just that the tangent space and the variety are the same dimension. We say that is nonsingular if it is nonsingular at every point . If you’ll grant me, just for a moment, the implicit function theorem from analysis, then we can prove (though we won’t, because the proof isn’t terribly illuminating) that any nonsingular variety is, with the induced topology from the classical topology on or , a complex manifold.
Now knowing this, to show that any algebraic group is a Lie group, we must only show that it is nonsingular at every point. First we’ll look at automorphisms of varieties. An automorphism is just an isomorphism . Now, we can compose and invert these perfectly well, and so the isomorphisms of form a group, which we will call . We say that an automorphism group is transitive if for any , there exists such that . That is, the automorphisms can move any point to any other.
Now, if there is an automorphism with , then we must have that by the map , because we can invert do get , which gives that the map of tangent spaces is an isomorphism. Thus, any automorphism takes the nonsingular points to nonsingular points. So in particular, if is transitive, then if ANY point is nonsingular, all of them must be!
Later, when we talk about singularities a bit, we’ll prove that any variety has a nonempty open subset which is nonsingular, and so, in particular, has a nonsingular point. So this says that if the automorphism group is transitive, then the variety is nonsingular.
And now the final piece of the puzzle: the automorphism groups of algebraic groups are always transitive. To see this, we note that if we fix one of the points and look at given by , this must be an isomorphism, and so is an isomorphism. So let . Then takes , and so , so the automorphism group is transitive. So we now have that algebraic groups are nonsingular, and therefore manifolds. And so algebraic groups are Lie groups.
That’s all for tonight, I’m off until January, so there won’t be any new posts until then. Anyway, enjoy your holidays and a happy New Year to everyone.