arXiv subjects

Amadou Bah

Publications and source records attributed to Amadou Bah.

6 recordsLinked to original sources

arXiv2026

Uniform stratified vanishing and equidistribution on $\mathbb{G}_m^d$

We prove a uniform stratified generic vanishing theorem for perverse sheaves on $\mathbb{G}_m^d$ over finite fields, with constants depending only on the dimension and on the complexity \`a la Sawin. As a corollary, we prove an equidistribution theorem for $\mathbb{G}_m^d$ extending Katz's theorem for $\mathbb{G}_m$. The corollary applies to sequences of perverse sheaves of bounded complexity with common tannakian monodromy group, over finite fields of varying characteristic. As the cardinalities of the fields tend to infinity, the Frobenius conjugacy classes attached to the multiplicative characters equidistribute in the space of conjugacy classes of a maximal compact subgroup.

math.AG↗

arXiv2022

Mixels: Fabricating Interfaces using Programmable Magnetic Pixels

In this paper, we present Mixels, programmable magnetic pixels that can be rapidly fabricated using an electromagnetic printhead mounted on an off-the-shelve 3-axis CNC machine. The ability to program magnetic material pixel-wise with varying magnetic force enables Mixels to create new tangible, tactile, and haptic interfaces. To facilitate the creation of interactive objects with Mixels, we provide a user interface that lets users specify the high-level magnetic behavior and that then computes the underlying magnetic pixel assignments and fabrication instructions to program the magnetic surface. Our custom hardware add-on based on an electromagnetic printhead and hall effect sensor clips onto a standard 3-axis CNC machine and can both write and read magnetic pixel values from magnetic material. Our evaluation shows that our system can reliably program and read magnetic pixels of various strengths, that we can predict the behavior of two interacting magnetic surfaces before programming them, that our electromagnet is strong enough to create pixels that utilize the maximum magnetic strength of the material being programmed, and that this material remains magnetized when removed from the magnetic plotter.

cs.HC↗

arXiv2022

Variation of the Swan conductor of an $\mathbb{F}_{\ell}$-sheaf on a rigid annulus

Let $C=A(r, r')$ be a closed annulus of radii $r$ and $r'$ ($r < r' \in \mathbb{Q}_{\geq 0}$) over a complete discrete valuation field with algebraically closed residue field of characteristic $p>0$. To an étale sheaf of $\mathbb{F}_{\ell}$-modules $\mathcal{F}$ on $C$, ramified at most at a finite set of rigid points of $C$, we associate an Abbes-Saito Swan conductor function $\mathrm{sw}_{\mathrm{AS}}(\mathcal{F}, \cdot): [r, r']\cap \mathbb{Q}_{\geq 0} \to \mathbb{Q}$ which, for the variable $t$, measures the ramification of $\mathcal{F}\lvert C^{[t]}$ - the restriction of $\mathcal{F}$ to the sub-annulus $C^{[t]}$ of $C$ of radius $t$ with $0$-thickness - along the special fiber of the normalized integral model of $C^{[t]}$. We show that this function is continuous, convex and piecewise linear outside the radii of the ramification points of $\mathcal{F}$, with finitely many slopes which are all integers. For two distinct radii $t$ and $t'$ lying between consecutive radii of ramification points of $\mathcal{F}$, we compute the difference of the slopes of $\mathrm{sw}_{\mathrm{AS}}(\mathcal{F}, \cdot)$ at $t$ and $t'$ as the difference of the orders of the characteristic cycles of $\mathcal{F}$ at $t$ and $t'$.

math.AG↗

arXiv2022

Variation of the Swan conductor of an $\mathbb{F}_{\ell}$-sheaf on a rigid disc

This article studies the variation of the Swan conductor of a lisse étale sheaf of $\mathbb{F}_{\ell}$-modules $\mathcal{F}$ on the rigid unit disc $D$ over a complete discrete valuation field $K$ with algebraically closed residue field of characteristic $p\neq \ell$. We associate to $\mathcal{F}$ a function ${\rm sw}_{\rm AS}(\mathcal{F}, \cdot): \mathbb{Q}_{\geq 0}\to \mathbb{Q}$, defined with the Abbes-Saito logarithmic ramification filtration, which measures, at each $t\in \mathbb{Q}_{\geq 0}$, the ramification of the restriction of $\mathcal{F}$ to the subdisc of radius $t$ along the special fiber of the normalized integral model. We prove that this function is continuous and piecewise linear, with finitely many slopes which are all integers. We compute the slope at $t\in \mathbb{Q}_{\geq 0}$ in terms of a characteristic cycle associated to $\mathcal{F}$, a (power of a) logarithmic differential form defined by ramification theory.

math.AG↗

arXiv2019

Phase Transition in the One-bit Johnson-Lindenstrauss Lemma

The Johnson-Lindenstrauss Lemma (J-L Lemma) is a cornerstone of dimension reduction techniques. We study it in the one-bit context, namely we consider the unit sphere $ \mathbb S ^{N-1}$, with normalized geodesic metric, and map a finite set $ \mathbf{X} \subset \mathbb{S}^{N-1}$ into the Hamming cube $\mathbb{H}_m = \{0,1\}^m$, with normalized Hamming metric. We find that for $ 0< δ<1$, and $m>\frac{\ln n}{2δ^2}$ there is a $δ$-RIP from $\mathbf{X}$ into $\mathbb{H}_m$. This is surprising as the value of $ m$ is virtually identical to best known bound linear J-L Lemma. In both the linear and one-bit case, the maps are randomly constructed. We show that the probability of $B_m$ being a $δ$-RIP satisfies a phase transition. It passes from probability of nearly zero to nearly one with a very small change in $m$. Our proof relies on delicate properties of Bernoulli random variables.

math.FA↗

arXiv2018

A Non-iterative Parallelizable Eigenbasis Algorithm for Johnson Graphs

We present a new $O(k^2 \binom{n}{k}^2)$ method for generating an orthogonal basis of eigenvectors for the Johnson graph $J(n,k)$. Unlike standard methods for computing a full eigenbasis of sparse symmetric matrices, the algorithm presented here is non-iterative, and produces exact results under an infinite-precision computation model. In addition, our method is highly parallelizable; given access to unlimited parallel processors, the eigenbasis can be constructed in only $O(n)$ time given n and k. We also present an algorithm for computing projections onto the eigenspaces of $J(n,k)$ in parallel time $O(n)$.

cs.DS↗