Chapter 2: Problem 33
Factor \(Y^{3}-2 X Y^{2}+2 X^{2} Y+X^{3}\) into linear factors in \(\mathbb{C}[X, Y]\)
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Chapter 2: Problem 33
Factor \(Y^{3}-2 X Y^{2}+2 X^{2} Y+X^{3}\) into linear factors in \(\mathbb{C}[X, Y]\)
These are the key concepts you need to understand to accurately answer the question.
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Let \(f\) be a rational function on a variety \(V\). Let \(U=\\{P \in V \mid f\) is defined at P\\}. Then \(f\) defines a function from \(U\) to \(k\). Show that this function determines \(f\) uniquely. So a rational function may be considered as a type of function, but only on the complement of an algebraic subset of \(V\), not on \(V\) itself.
Let \(F\) be an irreducible polynomial in \(k[X, Y]\), and suppose \(F\) is monic in \(Y\) : \(F=Y^{n}+a_{1}(X) Y^{n-1}+\cdots\), with \(n>0 .\) Let \(V=V(F) \subset \mathbb{A}^{2}\). Show that the natural homomorphism from \(k[X]\) to \(\Gamma(V)=k[X, Y] /(F)\) is one-to-one, so that \(k[X]\) may be regarded as a subring of \(\Gamma(V)\); show that the residues \(\overline{1}, \bar{Y}, \ldots, \bar{Y}^{n-1}\) generate \(\Gamma(V)\) over \(k[X]\) as a module.
(a)] Let \(\varphi: \mathbb{A}^{1} \rightarrow V=V\left(Y^{2}-X^{3}\right) \subset \mathbb{A}^{2}\) be defined by \(\varphi(t)=\left(t^{2}, t^{3}\right)\). Show that although \(\varphi\) is a one-to-one, onto polynomial map, \(\varphi\) is not an isomorphism. (Hint:: \(\left.\tilde{\varphi}(\Gamma(V))=k\left[T^{2}, T^{3}\right] \subset k[T]=\Gamma\left(\mathbb{A}^{1}\right) .\right)\) (b) Let \(\varphi: \mathbb{A}^{1} \rightarrow V=V\left(Y^{2}-X^{2}(X+1)\right)\) be de fined by \(\varphi(t)=\left(t^{2}-1, t\left(t^{2}-1\right)\right)\). Show that \(\varphi\) is one-to-one and onto, except that \(\varphi(\pm 1)=(0,0)\)
Let \(I=(X, Y) \subset k[X, Y]\). Show that \(\operatorname{dim}_{k}\left(k[X, Y] / I^{n}\right)=1+2+\cdots+n=\frac{n(n+1)}{2}\).
Let \(W\) be a subvariety of a variety \(V\), and let \(I_{V}(W)\) be the ideal of \(\Gamma(V)\) corresponding to \(W\). (a) Show that every polynomial function on \(V\) restricts to a polynomial function on \(W\). (b) Show that the map from \(\Gamma(V)\) to \(\Gamma(W)\) defined in part (a) is a surjective homomorphism with kernel \(I_{V}(W)\), so that \(\Gamma(W)\) is isomorphic to \(\Gamma(V) / I_{V}(W)\)
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