ScalingStacks

Example 4.13 . [02PP]

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Example 4.13.

Let N=ℤ2N=\mathbb{Z}^{2}, (a,b)∈N(a,b)\in N with gcd⁡(a,b)=1\gcd(a,b)=1 and ι:Q↪N\iota\colon Q\hookrightarrow N the saturated sublattice generated by (a,b)(a,b). Let Σ\Sigma be the fan in NℝN_{\mathbb{R}} of Example 3.70. Then XΣ=ℙ2X_{\Sigma}=\mathbb{P}^{2} with projective coordinates (x0:x1:x2)(x_{0}:x_{1}:x_{2}). The fan induced in QℝQ_{\mathbb{R}} has three cones: ΣQ={ℝ≤0,{0},ℝ≥0}\Sigma_{Q}=\{\mathbb{R}_{\leq 0},\{0\},\mathbb{R}_{\geq 0}\}. Thus XΣQ=ℙ1X_{\Sigma_{Q}}=\mathbb{P}^{1}. Let p=(1:p1:p2)p=(1:p_{1}:p_{2}) be a point of XΣ,0​(K)X_{\Sigma,0}(K). Then φp,ι((1:t))=(1:p1ta:p2tb)\varphi_{p,\iota}((1:t))=(1:p_{1}t^{a}:p_{2}t^{b}). Therefore, YΣ,Q,pY_{\Sigma,Q,p} is the curve of equation

p2a​x0a​x1b−p1b​x0b​x2a=0.p_{2}^{a}x_{0}^{a}x_{1}^{b}-p_{1}^{b}x_{0}^{b}x_{2}^{a}=0.

In general, this curve is not normal. Hence it is not a toric variety.

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