I'm having trouble calculating singularities of varieties: when my professor covered it it was the last lecture of the course and a shade rushed.
I'm not sure if the definition I've been given is standard, so I will quote it to be safe: the tangent space of a projective variety $X$ at a point $a$ is $T_aX = a + \mbox{ker}(\mbox{Jac}(X))$.
A variety is smooth at $a \in X$ if $a$ lives in a unique irreducible component $x_i$ of $X$ and $\dim T_a(X) = \dim X_i$, where dimension of a variety has been defined to be the degree of the Hilbert polynomial of $X$. A projective variety is smooth if its affine cone is.
I tried to calculate a few examples and it all went very wrong.
Example: The Grassmannian $G(2,4)$ in its Plucker embedding is $V(X_{12} x_{34} - x_{13}x_{24}+ x_{14}x_{23}) \subset \mathbb{P}^5$
I calculated the Hilbert polynomial to be $\frac{1}{12}d^4+...$, so it has dimension 4 (as expected), but I get
$\mbox{Jac}(G(2,4))= [x_{34}, -x_{24}, x_{23}, x_{14}, -x_{13}, x_{12}]$
Which has rank 1 where $x \ne 0$, so nullity 5. So assumedly $\dim T_aX = \dim( a + \mbox{ker}(\mbox{Jac}(X))) = \dim \mbox{ker} \mbox{Jac}(X) = \mbox{nullity} (\mbox{Jac}(X))$.
Which isn't 4?
Which is a bit silly, as the Grassmannian is obviously smooth.
I'm probably going wrong somewhere, but I've gotten myself thoroughly confused. Any help would be greatly appreciated.
Thanks!