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The u-invariant of a field is the supremum of the dimensions of anisotropic quadratic forms over the field. We define corresponding u-invariants for hermitian and generalised quadratic forms over a division algebra with involution in characteristic 2 and investigate the relationships between them. We also investigate these invariants in the case of a quaternion algebra and in particular when this quaternion algebra is the unique quaternion division algebra over a field., 20 pages

Field patching, introduced by Harbater and Hartmann, and extended by the aforementioned authors and Krashen, has recently seen numerous applications. We present an extension of this technique to the setting of Berkovich analytic geometry and applications to the local-global principle.In particular, we show that this adaptation of patching can be applied to Berkovich analytic curves, and as a consequence obtain local-global principles over function fields of curves defined over complete ultrametric fields. Because of the connection between the points of a Berkovich analytic curve and the valuations that its function field can be endowed with, one of these local-global principles is given with respect to completions, thus evoking some similarity with more classical versions. As an application, we obtain local-global principles for quadratic forms and results on the u-invariant. These findings generalize those of Harbater, Hartmann and Krashen.As a starting point for higher-dimensional patching in the Berkovich setting, we show that this technique is applicable around certain fibers of a relative Berkovich analytic curve. As a consequence, we prove a local-global principle over the germs of meromorphic functions on said fibers. By showing that said germs of meromorphic functions are algebraic, we also obtain local-global principles over function fields of algebraic curves defined over a larger class of ultrametric fields.; Le recollement sur les corps, introduit par Harbater et Hartmann, et étendu par ces auteurs et Krashen, a récemment trouvé de nombreuses applications. Nous présentons ici une extension de cette technique au cadre de la géométrie analytique de Berkovich et des applications au principe local-global.Nous montrons que cette adaptation du recollement peut s'appliquer aux courbes analytiques de Berkovich, et par conséquent obtenons des principes locaux-globaux sur les corps de fonctions de courbes définies sur des corps ultramétriques complets. Grâce à la connexion entre les points d'une courbe analytique de Berkovich et les valuations dont on peut munir son corps de fonctions, nous obtenons un principe local-global par rapport à des complétés du corps de fonctions considéré, ce qui présente une ressemblance avec des versions plus classiques. En application, nous établissons des principes locaux-globaux dans le cas plus précis des formes quadratiques et en déduisons des bornes sur l'u-invariant de certains corps. Nos résultats généralisent ceux de Harbater, Hartmann et Krashen.Comme point de départ pour le recollement en dimension supérieure dans un cadre d'espaces de Berkovich, nous montrons que cette technique peut s'appliquer autour de certaines fibres d'une courbe analytique relative. Nous l'utilisons ensuite pour démontrer un principe local-global sur les germes des fonctions méromorphes sur ces fibres. En montrant que ces germes de fonctions méromorphes sont algébriques, nous obtenons aussi des principes locaux-globaux sur les corps de fonctions des courbes algébriques définies sur une famille plus vaste de corps ultramétriques.

We extend field patching to the setting of Berkovich analytic geometry and use it to prove a local-global principle over function fields of analytic curves with respect to completions. In the context of quadratic forms, we combine it with sufficient conditions for local isotropy over a Berkovich curve to obtain applications on the u-invariant. The patching method we adapt was introduced by Harbater and Hartmann in [18], and further developed by these two authors and Krashen in [19]. The results presented in this paper generalize those of [19] on the local-global principle and quadratic forms., Recollement sur les courbes de Berkovich et formes quadratiques. Nous étendons la technique de recollement sur les corps au cadre de la géométrie ana-lytique de Berkovich pour démontrer un principe local-global sur les corps de fonctions de courbes analytiques par rapport à certains de leurs complétés. Dans le contexte des formes quadratiques, nous le combinons avec des conditions suffisantes d'isotropie locale sur une courbe de Berkovich pour obtenir des applications au u-invariant. La méthode de recollement que nous adaptons a été introduite par Harbater et Hartmann dans [18], puis développée par ces deux auteurs et Krashen dans [19]. Dans ce texte, nous présentons des résultats sur le principe local-global et les formes quadratiques qui généralisent ceux de [19].

The relevance in a sampling theory of $U$ -invariant subspaces of a Hilbert space $\mathcal {H}$ , where $U$ denotes a unitary operator on $\mathcal {H}$ , is nowadays a recognized fact. Indeed, shift-invariant subspaces of $L^{2}(\mathbb {R})$ become a particular example; periodic extensions of finite signals also provide a remarkable example. As a consequence, the availability of an abstract $U$ -sampling theory becomes a useful tool to handle these problems. In this paper, we derive a sampling theory for finite dimensional multiple generated $U$ -invariant subspaces of a Hilbert space $\mathcal {H}$ . As the involved samples are identified as frame coefficients in a suitable euclidean space, the relevant mathematical technique is that of the finite frame theory. Since finite frames are nothing but spanning sets of vectors, the used technique naturally meets matrix analysis.

In this paper we consider a variational problem consisting of an energy functional defined by the integral, $$\begin{aligned} \mathbb {F}[u,\mathbf{X}] = \frac{1}{2}\int _{\mathbf{X}} \frac{|\nabla u|^2}{|u|^2} \,dx, \end{aligned}$$ and an associated mapping space, here, the space of incompressible Sobolev mappings of the symmetric annular domain in the Euclidean n-space $$\mathbf{X}= \lbrace x \in {\mathbb {R}}^n{:}\,a

Let K be a complete discretely valued field with residue field k. If char(K) = 0, char(k) = 2 and the 2-rank of k is d, we prove that there exists an integer N depending on d such that the u-invariant of any function field in one variable over K is bounded by N. The method of proof is via introducing the notion of uniform boundedness for the p-torsion of the Brauer group of a field and relating the uniform boundedness of the 2-torsion of the Brauer group to finiteness of the u-invariant. We prove that the 2-torsion of the Brauer group of function fields in one variable over K are uniformly bounded., 11 pages

Recently, a sampling theory for infinite dimensional U -invariant subspaces of a separable Hilbert space H where U denotes a unitary operator on H has been obtained. Thus, uniform average sampling for shift-invariant subspaces of L 2 ( R ) becomes a particular example. As in the general case it is possible to have finite dimensional U -invariant subspaces, the main aim of this paper is to derive a sampling theory for finite dimensional U -invariant subspaces of a separable Hilbert space H . Since the used samples are frame coefficients in a suitable euclidean space C N , the problem reduces to obtain dual frames with a U -invariance property.

We investigate diagonal forms of degree $d$ over the function field $F$ of a smooth projective $p$-adic curve: if a form is isotropic over the completion of $F$ with respect to each discrete valuation of $F$, then it is isotropic over certain fields $F_U$, $F_P$ and $F_p$. These fields appear naturally when applying the methodology of patching; $F$ is the inverse limit of the finite inverse system of fields $\{F_U,F_P,F_p\}$. Our observations complement some known bounds on the higher $u$-invariant of diagonal forms of degree $d$. We only consider diagonal forms of degree $d$ over fields of characteristic not dividing $d!$., Comment: Some small corrections/changes have been done with respect to the first version

We associate an Albert form to any pair of cyclic algebras of prime degree p over a field F with $${\text {char}}(F)=p$$ which coincides with the classical Albert form when $$p=2$$ . We prove that if every Albert form is isotropic, then $$H^4(F)=0$$ . As a result, we obtain that if F is a linked field with $${\text {char}}(F)=2$$ , then its u-invariant is either 0, 2, 4, or 8.

We characterize flat extensions of commutative rings satisfying the Weierstrass preparation theorem. Using this characterization we prove a variant of the Weierstrass preparation theorem for rings of functions on a normal curve over a complete local domain of dimension one. This generalizes recent works of Harbater, Hartmann and Krashen with a different method of proof.

Over a finitely generated field extension in m variables over a p-adic field, any quadratic form in more than 2m + 2 variables has a nontrivial zero. This bound is sharp. We extend this result to a wider class of fields. A key ingredient to our proofs is a recent result of Heath-Brown on systems of quadratic forms over p-adic fields.

Two Tychonoff spaces X and Y are said to be l-equivalent (u-equivalent) if Cp(X) and Cp(Y) are linearly (uniformly) homeomorphic. N. V. Velichko proved that countable Lindelöf number is preserved by the relation of l-equivalence. A. Bouziad strengthened this result and proved that any Lindelöf number is preserved by the relation of l-equivalence. In this paper it has been proved that the Lindelöf number greater than continuum is preserved by the relation of u-equivalence., 2000 Mathematics Subject Classification: 54C35, 54D20, 54C60.

We study several field invariants arising in quadratic form theory. Some of the invariants considered are of particular interest in the study of real fields, including the length, the u-invariant, and the (reduced) stability index. In this context we give a systematic account of valuation theoretic arguments that lead to lower bounds for these invariants.

The u-invariant of a field K of characteristic not 2 is the largest dimension of an anisotropic torsion quadratic form over K. We extend the recent theorem by Harbater, Hartmann, and Krashen on the u-invariant of a one-dimensional function field over a complete discretely valued field to the case where the residue field can be ordered.

We study sums of squares, quadratic forms, and related field invariants in a quadratic extension of the rational function field in one variable over a hereditarily pythagorean base field.

Let F be a field of characteristic not 2, and assume that F has finite reduced stability. Let K/F be any finite extension. We prove that if the general u-invariant u(F) is finite, then u(K) is finite.

In the late 1970s, Herbert Gross asked whether there exist fields admitting anisotropic quadratic spaces of arbitrarily large finite dimensions but none of infinite dimension. We construct examples of such fields and also discuss related problems in the theory of central simple algebras and in Milnor K-theory.

We show that every sum of squares in the three-variable Laurent series field $\mathbb{R}((x,y,z))$ is a sum of 4 squares, as was conjectured in a paper of Choi, Dai, Lam and Reznick in the 1980's. We obtain this result by proving that every sum of squares in a finite extension of $\mathbb{R}((x,y))$ is a sum of $3$ squares. It was already shown in Choi, Dai, Lam and Reznick's paper that every sum of squares in $\mathbb{R}((x,y))$ itself is a sum of two squares. We give a generalization of this result where $\mathbb{R}$ is replaced by an arbitrary real field. Our methods yield similar results about the $u$-invariant of fields of the same type., Comment: final version, major revisions in the style of writing (abstract and introduction rewritten) compared to v.1

This note is motivated by the problem of determining the u-invariant of a field F of characteristic different from two when it is known that $$u\left( {F\left( {\sqrt { - 1} } \right)} \right) = 4.$$ A criterion is given to decide whether u(F) ≤ 4 in this situation

We show that a quadratic form defined over the rational function field ℚ(x 1 , …, x n ) of dimension at least 4.2 n + 1 is isotropic over all fields ℚ p (x 1 , …, x n ), except for finitely many primes. Partial results concerning the u-invariant of p-adic function fields are also shown.

We study the notion of hermitian u-invariant. We give some estimates of the u-invariant of a division algebra with involution in terms of the u-invariant of some subalgebras stable under the involution. We also find some finiteness results for comparing the u-invariant of a division algebra with involution and that of its centre. Some results about the values of this invariant are also given. A description of the Tits index of some algebraic groups of classical type over Open image in new window is given as an application.

In this paper we construct a family of circle-like continua, each admitting a finest monotone map onto S 1 such that there exists a subset of point inverses which is homeomorphic to the Cantor set cross an interval. We then show how to realize some members of this family as the boundaries ∂U of bounded irreducible local Siegel disks U . These boundaries are geometrically rigid in the following sense: there exist arbitrarily small periodic homeomorphisms of the sphere, conformal on U , which keep U invariant. The embedding portion of this paper follows a flexible construction of Herman. These results provide a partial answer to a question of Rogers and a complete answer to a question of Brechner, Guay, and Mayer.

We prove certain weak versions of some celebrated results due to Alexander Vishik comparing rationality of algebraic cycles over the function field of a quadric and over the base field. The original proofs use Vishik’s symmetric operations in the algebraic cobordism theory and work only in characteristic 0. Our proofs use the modulo 2 Steenrod operations in the Chow theory and work in any characteristic ≠ 2. Our weak versions are still sufficient for existing applications. In particular, Vishik’s construction of fields of u-invariant 2r + 1, for r ≥ 3, is extended to arbitrary characteristic ≠ 2.

Let q be a quadratic form over a field K of characteristic different from 2. We investigate the properties of the smallest positive integer n such that −1 is a sum of n values represented by q in several situations. We relate this invariant (which is called the q-level of K) to other invariants of K such as the level, the u-invariant and the Pythagoras number of K. The problem of determining the numbers which can be realized as a q-level for particular q or K is studied. We also observe that the q-level naturally emerges when one tries to obtain a lower bound for the index of the subgroup of non-zero values represented by a Pfister form q. We highlight necessary and/or sufficient conditions for the q-level to be finite. Throughout the paper, special emphasis is given to the case where q is a Pfister form.

This paper is concerned with the characterization as frames of some sequences inU-invariant spaces of a separable Hilbert spaceℋwhereUdenotes an unitary operator defined onℋ; besides, the dual frames having the same form are also found. This general setting includes, in particular, shift-invariant or modulation-invariant subspaces inL2ℝ, where these frames are intimately related to the generalized sampling problem. We also deal with some related perturbation problems. In doing so, we need the unitary operatorUto belong to a continuous group of unitary operators.

In this paper we prove an integral representation formula for the inverse Fueter mapping theorem for monogenic functions defined on axially symmetric open sets U ⊆ ℝ n+1, i.e. on open sets U invariant under the action of SO(n), where n is an odd number. Every monogenic function on such an open set U can be written as a series of axially monogenic functions of degree k, i.e. functions of type $$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\smile}$}}{f} _k (x) = \left[ {A\left( {x_{0,\rho } } \right) + \underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{\omega } {\rm B}\left( {x_{0,\rho } } \right)} \right]\mathcal{P}_k (\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{x} )$$ , where A(x 0, ρ) and B(x 0, ρ) satisfy a suitable Vekua-type system and $$\mathcal{P}_k (\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{x} )$$ is a homogeneous monogenic polynomial of degree k. The Fueter mapping theorem says that given a holomorphic function f of a paravector variable defined on U, then the function $$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\smile}$}}{f} _k (x)\mathcal{P}_k (\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{x} )$$ given by $$\Delta ^{k + \tfrac{{n - 1}} {2}} \left( {f(x)\mathcal{P}_k (\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{x} )} \right) = \overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\smile}$}}{f} (x)\mathcal{P}_k (\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{x} )$$ is a monogenic function. The aim of this paper is to invert the Fueter mapping theorem by determining a holomorphic function f of a paravector variable in terms of $$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\smile}$}}{f} _k (x)\mathcal{P}_k (\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{x} )$$ . This result allows one to invert the Fueter mapping theorem for any monogenic function defined on an axially symmetric open set.

Let K be a complete discretely valued field and F the function field of a curve over K. If the characteristic of the residue field k of K is p > 0, then we give a bound for the Brauer p-simension of F in terms of the p-rank of k. If k is a perfect field of characteristic 2, we show that the u-invaraint of F is at most 8.

We prove that if the maximal dimension of an anisotropic homogeneous polynomial form of prime degree [Formula: see text] over a field [Formula: see text] with [Formula: see text] is a finite integer [Formula: see text] greater than 1 then the symbol length of [Formula: see text]-algebras of exponent [Formula: see text] over [Formula: see text] is bounded from above by [Formula: see text], and show that every two tensor products of symbol algebras of lengths [Formula: see text] and [Formula: see text] with [Formula: see text] can be modified so that they share a common slot. For [Formula: see text], we obtain an upper bound of [Formula: see text] for the symbol length, which is sharp when [Formula: see text].

This thesis is concerned with the study of some arithmetic properties of certain algebraic varieties which are ``simplest'' in some geometric sense and which are defined over fields of geometric type. It consists of three chapters. In the first chapter, which is independent of the other two, we consider the weak approximation property for a smooth projective rationally connecte d variety X defined over the function field K=k(C) of an algebraic curve C over a field k. Suppose that X admits a K-rational point. Using geometric methods we prove that X(K) is Zariski dense in X if k is a large field, and that under suitable hypotheses weak approximation with respect to a set of places of good reduction holds for X. When k is a finite field, we obtain weak approximation at any given place of good reduction for a smooth cubic surface over K as well as a zero-th order weak approximation result for higher dimensional cubic hypersurfaces over K.The second part of the thesis consists of the last two chapters, where we work over the fraction field K of a 2-dimensional, excellent, henselian local domain R whose residue field k is often assumed to be finite, and where we use more algebraic tools. We first study the ramification and the cyclicity of division algebras over such a field K. We show in particular that every Brauer class over K of order n, which is prime to the residue characteristic, has index dividing n^2, and that the cyclicity of a Brauer class of prime order can be tested locally over the completions of K with respect to discrete valuations. These results are used in the last chapter to study the arithmetic of quadratic forms over K. We prove that every quadratic form of rank \ge 9 over K has a nontrivial zero. When K is the fraction field of a power series ring A[[t]] over a complete discrete valuation ring A, we prove the local-global principle for quadratic forms of rank \ge 5 over K. For general K we prove the local-global principle for quadratic forms of rank 5. The local-global principle for quadratic forms of rank 6, 7 or 8 is still open in the general case.; Cette thèse se concentre sur l'étude de quelques propriétés arithmétiques de certaines variétés algébriques qui sont ``les plus simples'' en un sens géométrique et qui sont définies sur des corps de type géométrique. Elle se compose de trois chapitres. Dans le premier chapitre, indépendant des deux autres, on s'intéresse à la propriété d'approximation faible pour une variété projective lisse rationnellement connexe X définie sur le corps de fonctions K=k(C) d'une courbe algébrique C sur un corps k. Supposons que X possède un K-point rationnel. En utilisant des méthodes géométriques, on démontre que X(K) est Zariski dense dans X si k est un corps fertile, et que l'approximation faible en un certain ensemble de places de bonne réduction vaut pour X sous des hypothèses supplémentaires convenables. Lorsque k est un corps fini, on obtient l'approximation faible en une place quelconque de bonne réduction pour une surface cubique lisse sur K ainsi qu'un résultat sur l'approximation faible d'ordre zéro pour des hypersurfaces cubiques de dimension supérieure sur K.Les deux autres chapitres forment la seconde partie de la thèse, où on travaille sur le corps des fractions K d'un anneau intègre local R, hensélien, excellent de dimension 2 dont le corps résiduel k est souvent supposé fini et où on emploie des outils plus algébriques. On étudie d'abord la ramification et la cyclicité des algèbres à division sur un tel corps K. On démontre en particulier que toute classe de Brauer d'ordre n premier à la caractéristique résiduelle sur K est d'indice divisant n^2 et que la cyclicité d'une classe de Brauer d'ordre premier peut être testée localement sur les corps complétés par rapport aux valuations discrètes de K. Ces résultats sont appliqués dans le dernier chapitre pour étudier l'arithmétique des formes quadratiques sur K. On montre que toute forme quadratique de rang \ge 9 sur K possède un zéro non trivial. Si K est le corps des fractions d'un anneau de séries formelles A[[t]] sur un anneau de valuation discrète complet A, on a prouvé le principe local-global pour toute forme quadratique de rang \ge 5 sur K. Pour K général on a établi le principe local-global pour les formes de rang 5. Le cas des formes de rang 6,7 ou 8 est ouvert.

This thesis is concerned with the study of some arithmetic properties of certain algebraic varieties which are ``simplest'' in some geometric sense and which are defined over fields of geometric type. It consists of three chapters. In the first chapter, which is independent of the other two, we consider the weak approximation property for a smooth projective rationally connecte d variety X defined over the function field K=k(C) of an algebraic curve C over a field k. Suppose that X admits a K-rational point. Using geometric methods we prove that X(K) is Zariski dense in X if k is a large field, and that under suitable hypotheses weak approximation with respect to a set of places of good reduction holds for X. When k is a finite field, we obtain weak approximation at any given place of good reduction for a smooth cubic surface over K as well as a zero-th order weak approximation result for higher dimensional cubic hypersurfaces over K.The second part of the thesis consists of the last two chapters, where we work over the fraction field K of a 2-dimensional, excellent, henselian local domain R whose residue field k is often assumed to be finite, and where we use more algebraic tools. We first study the ramification and the cyclicity of division algebras over such a field K. We show in particular that every Brauer class over K of order n, which is prime to the residue characteristic, has index dividing n^2, and that the cyclicity of a Brauer class of prime order can be tested locally over the completions of K with respect to discrete valuations. These results are used in the last chapter to study the arithmetic of quadratic forms over K. We prove that every quadratic form of rank \ge 9 over K has a nontrivial zero. When K is the fraction field of a power series ring A[[t]] over a complete discrete valuation ring A, we prove the local-global principle for quadratic forms of rank \ge 5 over K. For general K we prove the local-global principle for quadratic forms of rank 5. The local-global principle for quadratic forms of rank 6, 7 or 8 is still open in the general case., Cette thèse se concentre sur l'étude de quelques propriétés arithmétiques de certaines variétés algébriques qui sont ``les plus simples'' en un sens géométrique et qui sont définies sur des corps de type géométrique. Elle se compose de trois chapitres. Dans le premier chapitre, indépendant des deux autres, on s'intéresse à la propriété d'approximation faible pour une variété projective lisse rationnellement connexe X définie sur le corps de fonctions K=k(C) d'une courbe algébrique C sur un corps k. Supposons que X possède un K-point rationnel. En utilisant des méthodes géométriques, on démontre que X(K) est Zariski dense dans X si k est un corps fertile, et que l'approximation faible en un certain ensemble de places de bonne réduction vaut pour X sous des hypothèses supplémentaires convenables. Lorsque k est un corps fini, on obtient l'approximation faible en une place quelconque de bonne réduction pour une surface cubique lisse sur K ainsi qu'un résultat sur l'approximation faible d'ordre zéro pour des hypersurfaces cubiques de dimension supérieure sur K.Les deux autres chapitres forment la seconde partie de la thèse, où on travaille sur le corps des fractions K d'un anneau intègre local R, hensélien, excellent de dimension 2 dont le corps résiduel k est souvent supposé fini et où on emploie des outils plus algébriques. On étudie d'abord la ramification et la cyclicité des algèbres à division sur un tel corps K. On démontre en particulier que toute classe de Brauer d'ordre n premier à la caractéristique résiduelle sur K est d'indice divisant n^2 et que la cyclicité d'une classe de Brauer d'ordre premier peut être testée localement sur les corps complétés par rapport aux valuations discrètes de K. Ces résultats sont appliqués dans le dernier chapitre pour étudier l'arithmétique des formes quadratiques sur K. On montre que toute forme quadratique de rang \ge 9 sur K possède un zéro non trivial. Si K est le corps des fractions d'un anneau de séries formelles A[[t]] sur un anneau de valuation discrète complet A, on a prouvé le principe local-global pour toute forme quadratique de rang \ge 5 sur K. Pour K général on a établi le principe local-global pour les formes de rang 5. Le cas des formes de rang 6,7 ou 8 est ouvert.

Given a U-invariant sampling scheme on an arbitrary Hilbert space ℋ. This paper characterizes atomic subspaces A of ℋ such that every signal x ∈ A can be reconstructed from its samples acquired with this sampling scheme. If signal recovery is possible a linear filter is derived which reconstructs the signal from the samples.