We work over $\mathbb C$. The cone $S$ is the hypersurface of degree $d$ in $\mathbb{P}^n$ such that there exists a point $x\in S$ such that the multiplicity of $x$ in $S$ is $d$. Now the moduli space of hypersurface of degree $d$ in $\mathbb{P}^n=\mathbb P(V)$ is $\mathbb P(\mathrm{Sym}^dV)$ where $\dim V=n+1$ and let the locus $\Phi_{n,d}\subset \mathbb P(\mathrm{Sym}^dV)$ be the locus of cones. Is there some information of $\Phi_{n,d}$, such as dimension or some more explicit description?

I know some information as follows:

Consider the bundle of principal parts: $$\mathcal{P}^{d-1}=p_*(q^*\mathscr{O}_{\mathbb{P}^n}(d)\otimes\mathscr{O}_{\mathbb{P}^n\times\mathbb{P}^n}/\mathscr{I}_{\Delta}^{d})$$ where $p,q:\mathbb{P}^n\times\mathbb{P}^n\to\mathbb{P}^n$ are two projections. Hence if $\Phi_{n,d}$ has expected dimension, then $[\Phi_{n,d}]=c_{\mathrm{top}}(\mathcal{P}^{d-1})$. By some calculation $\deg\Phi_{n,d}=\binom{\binom{n+d-1}{n}}{n}$. Is there some more information of $\Phi_{n,d}$?

Thank you very much!


1 Answer 1


Dimension is fairly obvious, structure, maybe less so, it depends on what you want. The space of cones with vertex at a specific $x \in {\mathbb P}^n$ is naturally identified with the space of degree $d$ hypersurfaces in ${\mathbb P}(T_x{\mathbb P}^n)$ (I use `${\mathbb P}$' in the classical sense, projective space of lines), which means $$ I:= \{(x,\text{cone hypersurface $X$ in ${\mathbb P}^n$}): \text{$x$ is a vertex of $X$}\} \simeq {\mathbb P}\mathrm{Sym}^d\Omega_{{\mathbb P}^n}. $$ Now, $I \subset {\mathbb P}^n \times \{\text{projective space of degree $d$ hypersurfaces in ${\mathbb P}^n$}\}$, and the image of $I$ by the projection to the second factor is precisely $\Phi_{n,d}$. It's easy to check that the projection map is birational on $I$ (edit: if $d>1$), so $\dim \Phi_{n,d} = \dim {\mathbb P}\mathrm{Sym}^d\Omega_{{\mathbb P}^n}$, which is $n+{n+d-1 \choose n-1}-1$ unless I miscounted. However, $\Phi_{n,d}$ is not isomorphic to ${\mathbb P}\mathrm{Sym}^d\Omega_{{\mathbb P}^n}$, it's some (possibly weird, I haven't really thought) blowdown of it, since there are cones with more than one vertex -- the most extreme example would be $d$ times a hyperplane, e.g. $\{X_0^d=0\}$ -- so its structure might be a bit confusing.

  • $\begingroup$ Thank you for your answer! Actually I want to consider the space of homogeneous polynomials of degree $d-1$ (as a linear subspace of $\mathbb P(\mathrm{Sym}^{d-1}V)$) generated by the partial derivatives of homogeneous polynomials of degree $d$. This will define a rational map to the Grassmannian which undefined at $\Phi_{n,d}$. I want to find more description to describe this rational map. $\endgroup$
    – DVL-WakeUp
    Commented May 26 at 7:01

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