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2 votes
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Degree three, codimension one subvarieties lying on a quadratic hypersurface

If the linear span of $V$ has dimension $n-1$, then $V$ is a cubic hypersurface in a hyperplane. Otherwise, $V$ is a variety of minimal degree, hence it is a cone over a linear section of $\mathbb{P}^ …
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3 votes
Accepted

Counting maximally tangent conics relative to a cubic

If $X$ is a cubic and $P \in X$ is a point such that there is totally tangent at $P$ conic then $$ 6P = 2H, $$ where $H$ is the restriction to $X$ of the line class of $\mathbb{P}^2$. Thus, the set of …
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10 votes
Accepted

Counting curves of degree 4 in $\mathbb{P}^{3}$

Zero. Indeed, if the intersection $Q_1 \cap Q_2$ of two quadrics is singular at $p_1$, then there is a quadric $Q$ in the pencil generated by $Q_1$ and $Q_2$ which is singular at $p_1$. On the other h …
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6 votes

Embedding $G(2,n)$ into $G(k,n)$

Let $V = k^n$. The map in question is the composition of the canonical map $$ f:G(2,V) \to G(k,S^{k-1}V) $$ given by the $(k-1)$-th symmetric power of the tautological bundle, and the (noncanonical) …
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