Paper 3, Section II, G

Algebraic Geometry | Part II, 2010

(i) Let XX be a curve, and pXp \in X be a smooth point on XX. Define what a local parameter at pp is.

Now let f:XYf: X \rightarrow Y be a rational map to a quasi-projective variety YY. Show that if YY is projective, ff extends to a morphism defined at pp.

Give an example where this fails if YY is not projective, and an example of a morphism f:C2\{0}P1f: \mathbb{C}^{2} \backslash\{0\} \rightarrow \mathbf{P}^{1} which does not extend to 0.0 .

(ii) Let V=Z(X08+X18+X28)V=Z\left(X_{0}^{8}+X_{1}^{8}+X_{2}^{8}\right) and W=Z(X04+X14+X24)W=Z\left(X_{0}^{4}+X_{1}^{4}+X_{2}^{4}\right) be curves in P2\mathbf{P}^{2} over a field of characteristic not equal to 2 . Let ϕ:VW\phi: V \rightarrow W be the map [X0:X1:X2][X02:X12:X22]\left[X_{0}: X_{1}: X_{2}\right] \mapsto\left[X_{0}^{2}: X_{1}^{2}: X_{2}^{2}\right]. Determine the degree of ϕ\phi, and the ramification epe_{p} for all pVp \in V.

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