Tangent Vectors at a Point
Tangent vectors are a very important concept in differential geometry, and they’re one of the biggest stumbling blocks in comprehension. There are two major approaches: one more geometric, and one more algebraic. I find the algebraic approach a bit more satisfying, since it gets straight into the important properties of tangent vectors and how they are used, and it helps set the stage for tangent vectors in other contexts like algebraic geometry. Unfortunately, it’s not at all clear at first what this definition means geometrically, and why these things deserve being called “tangent vectors”. So I have to ask a little patience.
Now, we take a manifold with structure sheaf . We pick some point and get the stalk of germs of functions at . This is a real algebra, and we define a “tangent vector at ” to be a “derivation at ” of this algebra. That is, is a function satisfying
Indeed, let’s take a point and consider the operation defined by for any function that is differentiable at . This is linear, since both the derivative and evaluation operations are linear. The product rule tells us that
So satisfies the definition of a “tangent vector at “. Indeed, as it turns out corresponds to what we might normally consider the vector based at pointing one unit in the positive direction.
It should immediately be clear that the tangent vectors at form a vector space. Indeed, the sum of two tangent vectors at is firstly the sum of two linear functionals, which is again a linear functional. To see that it also satisfies the “derivation” condition, let and be tangent vectors at and check
Checking that scalar multiples of tangent vectors at are again tangent vectors at is similar. We write to denote this vector space of tangent vectors at to the manifold .
I want to call attention to one point of notation here, and I won’t really bother with it again. We seem to be using each of and to refer to two different things: a germ in — which is an equivalence class of sorts — and some actual function in for some neighborhood of which represents the germ. To an extent we are, and the usual excuse is that since we only ever evaluate the function at itself, it doesn’t really matter which representative of the germ we pick.
However, a more nuanced view will see that we’ve actually overloaded the notation . Normally this would mean evaluating a function at a point, yes, but here we interpret it in terms of the local ring structure of . Given a germ there is a projection , which we write as .
If all this seems complicated, don’t really worry about it. You can forget the whole last paragraph and get by on “sometimes we use a germ as if it’s an actual function defined in a neighborhood of , and it will never matter which specific representative function we use because we only ever ask what happens at itself.”