The topology of the amoeba map and maximal volume

For a smooth complete intersection $V\subset(\C^*)^n$ of dimension $k$, $n\ge2k$, we bound the volume of its amoeba by $π^{2k}$ times a sum of mixed volumes, divided by the order of any finite group $Γ$ of symmetries commuting with the logarithmic map. For $n=2k\ge4$ almost every fibre of the amoeba map has a number of points divisible by $2|Γ|$. For $n=2k$ we characterise the varieties of maximal volume, show that for $k\ge2$ they are not real, and prove that no plane in general position in $(\C^*)^4$ has maximal volume. For $n>2k$ the bound is never attained, varieties of maximal volume have amoeba maps injective off a null set, and injectivity is generic. The amoeba map of almost every real $k$-plane is two-to-one, and that of the $k$-th symmetric power of a real line exactly $2^k$-to-one; conversely, real $k$-planes in general position in $(\C^*)^{2k}$ with $2^k$-to-one amoeba maps are symmetric powers. We also classify the real planes of $(\C^*)^4$ and study lines in $(\C^*)^3$.

Authors

Publication Details

Journal
arXiv (Cornell University)
Published
2026-11-02
DOI
https://doi.org/10.48550/arxiv.2611.02496
Primary Topic
Algebraic Geometry and Number Theory
Type
preprint
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preprint

The topology of the amoeba map and maximal volume

Mounir Nisse
arXiv (Cornell University)
Algebraic Geometry and Number Theory
preprint

The topology of the amoeba map and maximal volume

Mounir Nisse
preprint en

Abstract

For a smooth complete intersection $V\subset(\C^*)^n$ of dimension $k$, $n\ge2k$, we bound the volume of its amoeba by $π^{2k}$ times a sum of mixed volumes, divided by the order of any finite group $Γ$ of symmetries commuting with the logarithmic map. For $n=2k\ge4$ almost every fibre of the amoeba map has a number of points divisible by $2|Γ|$. For $n=2k$ we characterise the varieties of maximal volume, show that for $k\ge2$ they are not real, and prove that no plane in general position in $(\C^*)^4$ has maximal volume. For $n>2k$ the bound is never attained, varieties of maximal volume have amoeba maps injective off a null set, and injectivity is generic. The amoeba map of almost every real $k$-plane is two-to-one, and that of the $k$-th symmetric power of a real line exactly $2^k$-to-one; conversely, real $k$-planes in general position in $(\C^*)^{2k}$ with $2^k$-to-one amoeba maps are symmetric powers. We also classify the real planes of $(\C^*)^4$ and study lines in $(\C^*)^3$.

arXiv (Cornell University)
Algebraic Geometry and Number Theory
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