ScalingStacks

Example 5.5 (The amoeba) . [04JX]

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Example 5.5 (The amoeba).

Take as a symplectomorphism Φ\Phi the linear map

Φ⁡(u1,u2)=12​(u1−u2,u1+u2−2).\Phi(u_{1},u_{2})=\frac{1}{\sqrt{2}}\left(u_{1}-u_{2},u_{1}+u_{2}-\sqrt{2}\right). (34)

Then the fibration resulting from Proposition 5.4 can be written explicitly in the coordinates of the total space. We obtain:

f⁡(z1,z2,z3)=(12​(|z1|2−|z2|2),log⁡12​|γ−z3|,log⁡12​|γ+z3−2|),f(z_{1},z_{2},z_{3})=\left(\frac{1}{2}\left(|z_{1}|^{2}-|z_{2}|^{2}\right),\log\frac{1}{\sqrt{2}}\left|\gamma-z_{3}\right|,\log\frac{1}{\sqrt{2}}\left|\gamma+z_{3}-\sqrt{2}\right|\right), (35)

where γ\gamma is as in (33). It is not difficult to see that Φ∘Γ0\Phi\circ\Gamma_{0} sends Σ\Sigma to the surface in (ℂ∗)2(\mathbb{C}^{\ast})^{2} given by

Σ′={v1+v2+1=0},\Sigma^{\prime}=\{v_{1}+v_{2}+1=0\},

which is, topologically, a pair of pants. Then the discriminant locus is

Δ={0}×Log⁡(Σ′),\Delta=\{0\}\times\Log(\Sigma^{\prime}),

which has the shape in Figure 4. This example is topologically conjugate to the one in Example 2.9, before the surface Σ′\Sigma^{\prime} has been twisted. For the discussion of the topology of the fibres in this example we refer to Example 2.9.

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