Chapter 2: Problem 20
Find the dual curve of \(x_{0}^{3}+x_{1}^{3}+x_{2}^{3}=0\).
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Chapter 2: Problem 20
Find the dual curve of \(x_{0}^{3}+x_{1}^{3}+x_{2}^{3}=0\).
These are the key concepts you need to understand to accurately answer the question.
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Let \(X\) be an affine variety and \(K\) a finite extension of \(k(X)\). Prove that there exists an affine variety \(Y\) and a map \(f: Y \rightarrow X\) with the properties (1) \(f\) is finite; (2) \(Y\) is normal; (3) \(k(Y)=K\) with \(f^{*}: k(X) \hookrightarrow k(Y)=K\) the given inclusion. Prove that \(Y\) is uniquely determined by these properties. It is called the normalisation of \(X\) in \(K\).
Let \(V \subset \mathbb{A}^{3}\) be the quadratic cone defined by \(x y=z^{2}\); let \(X^{\prime} \rightarrow \mathbb{A}^{3}\) be the blowup of \(\mathbb{A}^{3}\) with centre in the origin, and \(V^{\prime}\) the closure of \(\sigma^{-1}(V \backslash 0)\) in \(X^{\prime}\). Prove that \(V^{\prime}\) is a nonsingular variety and that the inverse image of the origin under \(\sigma: V^{\prime} \rightarrow V\) is a nonsingular rational curve.
Prove that if \(X \subset \mathbb{P}^{n}\) is a nonsingular hypersurface and not a hyperplane, then as \(x\) runs through \(X\), the tangent hyperplanes \(\Theta_{x}\) form a hypersurface in the dual space \(\mathbb{P}^{n *}\)
For what values of \(a\) does the curve \(x_{0}^{3}+x_{1}^{3}+x_{2}^{3}+a\left(x_{0}+x_{1}+x_{2}\right)^{3}=0\) have a singular point? What are its singular points then? Is it reducible?
Suppose that a nonsingular irreducible affine \(n\)-dimensional variety \(X \subset \mathbb{A}^{n}\) is given by equations \(F_{1}=\cdots=F_{m}=0\), and that for every \(x=\left(x_{1}, \ldots, x_{n}\right) \in X\) the space defined by \(\sum\left(\partial F_{i} / \partial T_{j}\right)(x)\left(T_{j}-x_{j}\right)=0\) is \(n\)-dimensional. Prove that then \(\mathfrak{a}_{X}=\left(F_{1}, \ldots, F_{m}\right)\)
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