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Ahmed Laghribi

Publications and source records attributed to Ahmed Laghribi.

12 recordsLinked to original sources
arXiv2026

Sums of two symbols in $K_2(F)/2K_2(F)$ in characteristic two

In this paper, study sums $A=\{a,b\}_2+\{c,d\}_2$ of two symbols in $K_2(F)/2K_2(F)$ when $\operatorname{char}(F)=2$. We first prove a chain lemma that connects $A$ to $B=\{α,β\}_2+\{γ,δ\}_2$ by a finite sequence of small steps when $A \equiv B$. We use this lemma to prove that $\{a,b,c,d\}_2 \in K_4(F)/2K_4(F)$ is a well-defined invariant of $A$, and that this invariant is trivial if and only if $A$ is congruent to a single symbol in $K_2(F)/4K_2(F)$. We also bound the symbol length of $C$ in $K_2(F)/2^m K_2(F)$ from above when $C$ is the sum of up to four symbols in $K_2(F)/2^{m+1}K_2(F)$.

math.KT↗
arXiv2026

A cohomological invariant for algebras of degree 8 and exponent 2 in characteristic 2

Our aim in this paper is to extend a work of Sivatski to characteristic 2. More precisely, for $F$ a field of characteristic $2$ and a central simple algebra $A$ of exponent 2 that splits over a triquadratic extension of $F$ of separability degree at least 4, we attach a cohomological invariant $\inv(A) \in H_2^3(F) / G$, where $H_2^3(F)$ is the third Kato-Milno cohomology group and $G$ is a subgroup of $H_2^3(F)$ divisible by the Brauer class of $A$. As an application, we will relate the decomposability of the algebra in degree 8 to the vanishing of $\inv(A)$. Moreover, we will use this invariant to prove some descent results for central simple algebras and quadratic forms over biquadratic extensions.

math.NT↗
arXiv2025

Essential dimension of sequences of quadratic Pfister forms

We study the essential dimension of the set of isometry classes of $m$-tuples $(φ_1,...,φ_m)$ of quadratic $n$-fold Pfister forms over a field $F$ such that the Witt class of $φ_1 \perp \ldots \perp φ_m$ lies in $I_q^{n+1}F$. We show that the essential dimension is equal to $n+1$, when $m=3$, and is either $4$ or $5$, when $n=\text{char} F=2$, $m=4$.

math.NT↗
arXiv2025

Mixed multiquadratic splitting fields

We study mixed multiquadratic field extensions as splitting fields for central simple algebras of exponent $2$ in characteristic $2$. As an application, we provide examples of nonexcellent mixed biquadratic field extensions.

math.NT↗
arXiv2025

5-dimensional minimal quadratic and bilinear forms over function fields of conics

Over a field of characteristic 2, we give a complete classification of quadratic and bilinear forms of dimension 5 that are minimal over the function field of an arbitrary conic. This completes the unique known case due to Faivre concerning the classification of minimal quadratic forms of dimension 5 and type (2,1) over function fields of nonsingular conics.

math.AC↗
arXiv2025

Kato-Milne cohomology group over rational function fields in characteristic 2, I

Let F be a field of characteristic 2. In this paper we determine the Kato-Milne cohomology of the rational function field F(x) in one variable x. This will be done by proving an analogue of the Milnor exact sequence [4] in the setting of Kato-Milne cohomology. As an application, we answer the open case of the norm theorem for Kato-Milne cohomology that concerns separable irreducible polynomials in many variables. This completes a result of Mukhija [17, Theorem A.3] that gives the norm theorem for inseparable polynomials.

math.AC↗
arXiv2023

Chow Groups of Quadrics in Characteristic Two

Let $X$ be a smooth projective quadric defined over a field of characteristic 2. We prove that in the Chow group of codimension 2 or 3 of $X$ the torsion subgroup has at most two elements. In codimension 2, we determine precisely when this torsion subgroup is nontrivial. In codimension 3, we show that there is no torsion if $\dim X\ge 11$. This extends the analogous results in characteristic different from 2, obtained by Karpenko in the nineteen-nineties.

math.NT↗
arXiv2022

Autour de la décomposition des algèbres d'exposant 2 sur les extensions multiquadratiques

For central simple algebras of exponent $2$ over fields of characteristic $2$ and $2$-cohomological dimension equal to $2$, we study the adapted decomposition to some multiquadratic extensions of the base field. Several remarkable properties are extended to multiquadratic extensions of separability degree at most $4$. We also extend to the characteristic $2$ a result of Elman-Lam-Tignol-Wadsworth by constructing an algebra of exponent $2$ and degree $8$ containing a separable triquadratic extension but which admits no adapted decomposition to this extension. As an application we give an elementary proof of the non-excellence of separable biquadratic extensions.

math.RA↗
arXiv2019

The descent of biquaternion algebras in characteristic two

In this paper we associate an invariant to a biquaternion algebra $B$ over a field $K$ with a subfield $F$ such that $K/F$ is a quadratic separable extension and $\operatorname{char}(F)=2$. We show that this invariant is trivial exactly when $B \cong B_0 \otimes K$ for some biquaternion algebra $B_0$ over $F$. We also study the behavior of this invariant under certain field extensions and provide several interesting examples.

math.AC↗
arXiv2016

Total linkage of quaternion algebras and Pfister forms in characteristic two

We study the subfields of quaternion algebras that are quadratic extensions of their center in characteristic 2. We provide examples of the following: two non-isomorphic quaternion algebras that share all their quadratic subfields, two quaternion algebras that share all their inseparable but not all their separable quadratic subfields and two algebras that share all their separable but not all their inseparable quadratic subfields. We also discuss quaternion algebras over global fields and fields of Laurent series over a perfect field of characteristic 2 and show that the quaternion algebras over these fields are determined by their separable quadratic subfields. Throughout, these linkage questions are treated in the more general setting by considering the linkage of Pfister forms.

math.RA↗