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5.2. Non-Archimedian amoebas [04T3]

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5.2. Non-Archimedian amoebas

If V⊂(ℂ∗)n+1V\subset(\mathbb{C}^{*})^{n+1} be an algebraic variety. The image Log⁡(V)⊂(ℂ∗)n+1\operatorname{Log}(V)\subset(\mathbb{C}^{*})^{n+1} is called the amoeba of VV, see [4]. Note that amoebas make sense also for varieties over other fields KK as long as we have a norm K∗=K∖{0}→ℝ+K^{*}=K\smallsetminus\{0\}\to\mathbb{R}_{+}. The map LogK:(K∗)n+1→ℝn+1\operatorname{Log}_{K}:(K^{*})^{n+1}\to\mathbb{R}^{n+1} is defined by LogK⁡(z1,…,zn+1)=(log⁡‖z1‖K,…,log⁡‖zn+1‖)\operatorname{Log}_{K}(z_{1},\dots,z_{n+1})=(\log||z_{1}||_{K},\dots,\log||z_{n+1}||) and the amoeba of VK⊂(K∗)n+1V_{K}\subset(K^{*})^{n+1} is defined to be LogK⁡(VK)\operatorname{Log}_{K}(V_{K}).

A particularly useful case is when KK is an algebraically closed field with a non-Archimedian valuation. Recall that a non-Archimedian valuation is a function val:K∗→ℝ\operatorname{val}:K^{*}\to\mathbb{R} 22 2 Sometimes a valuation is defined as minus such a function. such that val⁡(a+b)≤max⁡{val⁡(a),val⁡(b)}\operatorname{val}(a+b)\leq\max\{\operatorname{val}(a),\operatorname{val}(b)\} and val⁡(a​b)=val⁡(a)+val⁡(b)\operatorname{val}(ab)=\operatorname{val}(a)+\operatorname{val}(b). Note that evale^{\operatorname{val}} gives a norm on KK and LogK\operatorname{Log}_{K} is nothing but taking the coordinatewise valuation.

Non-Archimedian amoebas of hypersurfaces were completely described in [7]. An example of such field is the field KK of the Puiseux series with complex coefficients in tt. Namely an element of KK is a formal series b⁡(t)=∑k∈Jbk​tkb(t)=\sum\limits_{k\in J}b_{k}t^{k}, bk∈ℂ∗b_{k}\in\mathbb{C}^{*} where J⊂ℝJ\subset\mathbb{R} is any bounded from below set contained in a finite union of arithmetic progressions. The valuation is defined by val⁡‖b⁡(t)‖=−min⁡J\operatorname{val}||b(t)||=-\min J. Note that we used irrational as well as rational powers in the Puiseux series to make the valuation surjective.

Theorem (Kapranov [7]).

If VK⊂(K∗)n+1V_{K}\subset(K^{*})^{n+1} is a hypersurface given by a polynomial f=∑aj​zjf=\sum a_{j}z^{j}, aj∈K∗a_{j}\in K^{*} then the (non-Archimedian) amoeba of VKV_{K} is a balanced polyhedral complex corresponding to the function v⁡(j)=val⁡(aj)v(j)=\operatorname{val}(a_{j}) defined on the lattice points of the Newton polyhedron Δ\Delta of VKV_{K} as in Example 2.

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