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Documents  Godefroy, Gilles | enregistrements trouvés : 16

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Research talks;Analysis and its Applications

Nigel Kalton played a prominent role in the development of a holomorphic functional calculus for unbounded sectorial operators. He showed, in particular, that such a calculus is highly unstable under perturbation: given an operator $D$ with a bounded functional calculus, fairly stringent conditions have to be imposed on a perturbation $B$ for $DB$ to also have a bounded functional calculus. Nigel, however, often mentioned that, while these results give a fairly complete picture of what is true at a pure operator theoretic level, more should be true for special classes of differential operators. In this talk, I will briefly review Nigel's general results before focusing on differential operators with perturbed coefficients acting on $L_p(\mathbb{R}^{n})$. I will present, in particular, recent joint work with $D$. Frey and A. McIntosh that demonstrates how stable the functional calculus is in this case. The emphasis will be on trying, as suggested by Nigel, to understand what makes differential operators so special from an operator theoretic point of view. Nigel Kalton played a prominent role in the development of a holomorphic functional calculus for unbounded sectorial operators. He showed, in particular, that such a calculus is highly unstable under perturbation: given an operator $D$ with a bounded functional calculus, fairly stringent conditions have to be imposed on a perturbation $B$ for $DB$ to also have a bounded functional calculus. Nigel, however, often mentioned that, while these ...

47F05 ; 47A60 ; 42B30 ; 42B37

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Research talks;Analysis and its Applications

46B85 ; 20F65 ; 30L05

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Research talks;Analysis and its Applications

One of my recent main interests has been the characterization of boundedness of (integral) operators between two $L^p$ spaces equipped with two different measures. Some recent developments have indicated a need of "Banach spaces and their applications" also in this area of Classical Analysis. For instance, while the theory of two-weight $L^2$ inequalities is already rich enough to deal with a number of singular operators (like the Hilbert transform), the $L^p$ theory has been essentially restricted to positive operators so far. In fact, a counterexample of $F$. Nazarov shows that the common "Sawyer testing" or "David-Journé $T(1)$" type characterization will fail, in general, in the two-weight $L^p$ world. What comes to rescue is what we so often need to save the $L^2$ results in an Lp setting: $R$-boundedness in place of boundedness! Even in the case of positive operators, it turns out that a version of "sequential boundedness" is useful to describe the boundedness of operators from $L^p$ to $L^q$ when $q < p$. - This is about my recent joint work with T. Hänninen and K. Li, as well as the work of my student E. Vuorinen.

two-weight inequalities - boundedness - singular operators
One of my recent main interests has been the characterization of boundedness of (integral) operators between two $L^p$ spaces equipped with two different measures. Some recent developments have indicated a need of "Banach spaces and their applications" also in this area of Classical Analysis. For instance, while the theory of two-weight $L^2$ inequalities is already rich enough to deal with a number of singular operators (like the Hilbert ...

42B25 ; 47G40

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Research talks;Analysis and its Applications

I'd like to share with the audience the Kaltonian story behind [1], started in 2004, including the problems we wanted to solve, and could not.
In that paper we show that Rochberg's generalized interpolation spaces $\mathbb{Z}^{(n)}$ [5] can be arranged to form exact sequences $0\to\mathbb{Z}^{(n)}\to\mathbb{Z}^{(n+k)}\to\mathbb{Z}^{(k)} \to 0$. In the particular case of Hilbert spaces obtained from the interpolation scale of $\ell_p$ spaces then $\mathbb{Z}^{(2)}$ becomes the well-known Kalton-Peck $Z_2$ space, and one gets from here that there are quite natural nontrivial twisted sums $0\to Z_2\to\mathbb{Z}^{(4)}\to Z_2 \to0$ of $Z_2$ with itself. The twisted sum space $\mathbb{Z}^{(4)}$ does not embeds in, or is a quotient of, a twisted Hilbert space and does not contain $\ell_2$ complemented. We will also construct another nontrivial twisted sum of $Z_2$ with itself that contains $\ell_2$ complemented. These results have some connection with the nowadays called Kalton calculus [3, 4], and thus several recent advances [2] in this theory that combines twisted sums and interpolation theory will be shown.

Banach space - twisted sum - complex interpolation - Hilbert space
I'd like to share with the audience the Kaltonian story behind [1], started in 2004, including the problems we wanted to solve, and could not.
In that paper we show that Rochberg's generalized interpolation spaces $\mathbb{Z}^{(n)}$ [5] can be arranged to form exact sequences $0\to\mathbb{Z}^{(n)}\to\mathbb{Z}^{(n+k)}\to\mathbb{Z}^{(k)} \to 0$. In the particular case of Hilbert spaces obtained from the interpolation scale of $\ell_p$ spaces then ...

46M18 ; 46B70 ; 46B20

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Research talks;Analysis and its Applications

We study the Daugavet equation
$\parallel Id+T\parallel$ $=1$ $+$ $\parallel T\parallel$
for Lipschitz operators on a Banach space. For this we introduce a substitute for the concept of slice for the case of non-linear Lipschitz functionals and transfer some results about the Daugavet and the alternative Daugavet equations previously known only for linear operators to the non-linear case.

numerical radius - numerical index - Daugavet equation - Daugavet property - SCD space - Lipschitz operator
We study the Daugavet equation
$\parallel Id+T\parallel$ $=1$ $+$ $\parallel T\parallel$
for Lipschitz operators on a Banach space. For this we introduce a substitute for the concept of slice for the case of non-linear Lipschitz functionals and transfer some results about the Daugavet and the alternative Daugavet equations previously known only for linear operators to the non-linear case.

numerical radius - numerical index - Daugavet equation - ...

46B04 ; 46B80 ; 46B22 ; 47A12

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Research talks;Analysis and its Applications

I shall discuss the theory of multi-norms. This has connections with norms on tensor products and with absolutely summing operators. There are many examples, some of which will be mentioned. In particular we shall describe multi-norms based on Banach lattices, define multi-bounded operators, and explain their connections with regular operators on lattices. We have new results on the equivalences of multi-norms. The theory of decompositions of Banach lattices with respect to the canonical 'Banach-lattice multi-norm' has a pleasing form because of a substantial theorem of Nigel Kalton that I shall state and discuss. I shall also discuss brie y a generalization that gives 'pmulti-norms' (for $1\leq p\leq1$) and an extension of a representation theorem of Pisier that shows that many pmulti-norms are 'sous-espaces de treillis'. The theory is based on joint work with Maxim Polyakov (deceasead), Hung Le Pham (Wellington), Matt Daws (Leeds), Paul Ramsden (Leeds), Oscar Blasco (Valencia), Niels Laustsen (Lancaster), Timur Oikhberg (Illinois), and Vladimir Troitsky (Edmonton).

multi-norms - equivalences - absolutely summing operators - tensor products
I shall discuss the theory of multi-norms. This has connections with norms on tensor products and with absolutely summing operators. There are many examples, some of which will be mentioned. In particular we shall describe multi-norms based on Banach lattices, define multi-bounded operators, and explain their connections with regular operators on lattices. We have new results on the equivalences of multi-norms. The theory of decompositions of ...

46B15 ; 46B28 ; 46B42 ; 47L10

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Research talks;Analysis and its Applications

I’ll discuss the Banach algebra structure of the spaces of bounded linear operators on $\ell_p$ and $L_p$ := $L_p(0, 1)$. The main new results are
1. The only non trivial closed ideal in $L(L_p)$, 1 $\leq$ p < $\infty$, that has a left approximate identity is the ideal of compact operators (joint with N. C. Phillips and G. Schechtman).
2. There are infinitely many; in fact, a continuum; of closed ideals in $L(L_1)$ (joint with G. Pisier and G. Schechtman).
The second result answers a question from the 1978 book of A. Pietsch, “Operator ideals”.
I’ll discuss the Banach algebra structure of the spaces of bounded linear operators on $\ell_p$ and $L_p$ := $L_p(0, 1)$. The main new results are
1. The only non trivial closed ideal in $L(L_p)$, 1 $\leq$ p < $\infty$, that has a left approximate identity is the ideal of compact operators (joint with N. C. Phillips and G. Schechtman).
2. There are infinitely many; in fact, a continuum; of closed ideals in $L(L_1)$ (joint with G. Pisier and G. ...

47L10 ; 47B10 ; 46E30

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Research talks;Analysis and its Applications;Geometry

In a recent paper, the speaker and M.I. Ostrovskii developed a new metric embedding method based on the theory of equal-signs-additive (ESA) sequences developed by Brunel and Sucheston in 1970’s. This method was used to construct bilipschitz embeddings of diamond and Laakso graphs with an arbitrary finite number of branches into any non-superreflexive Banach space with a uniform bound on distortions that is independent of the number of branches.
In this talk we will outline a proof that the above mentioned embeddability results cannot be obtained using the embedding method which was used for trees by Bourgain (1986) and for binary branching diamonds and Laakso graphs by Johnson and Schechtman (2009), and which is based on a classical James’ characterization of superreflexivity (the factorization between the summing basis and the unit vector basis of $\ell_1$). Our proof uses a “self-improvement” argument and the Ramsey theorem.
Joint work with M.I. Ostrovskii.
In a recent paper, the speaker and M.I. Ostrovskii developed a new metric embedding method based on the theory of equal-signs-additive (ESA) sequences developed by Brunel and Sucheston in 1970’s. This method was used to construct bilipschitz embeddings of diamond and Laakso graphs with an arbitrary finite number of branches into any non-superreflexive Banach space with a uniform bound on distortions that is independent of the number of ...

46B85 ; 05C12 ; 30L05 ; 46B07 ; 46B10

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Research talks;Analysis and its Applications;Geometry

The study of groups often sheds light on problems in various areas of mathematics. Whether playing the role of certain invariants in topology, or encoding symmetries in geometry, groups help us understand many mathematical objects in greater depth. In coarse geometry, one can use groups to construct examples or counterexamples with interesting or surprising properties. In this talk, we will introduce one such metric object arising from finite quotients of finitely generated groups, and survey some of its useful properties and associated constructions. The study of groups often sheds light on problems in various areas of mathematics. Whether playing the role of certain invariants in topology, or encoding symmetries in geometry, groups help us understand many mathematical objects in greater depth. In coarse geometry, one can use groups to construct examples or counterexamples with interesting or surprising properties. In this talk, we will introduce one such metric object arising from finite ...

46B85 ; 20F65 ; 20F69

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Research talks;Analysis and its Applications;Geometry

Exploring the relations between algebraic and geometric properties of a group and the geometry of the Banach spaces on which it can act is a fascinating program, still widely mysterious, and which is tightly connected to coarse embeddability of graphs into Banach spaces. I will present a recent contribution, joint with Tim de Laat, where we give a spectral (hilbertian) criterion for fixed point properties on uniformly curved Banach spaces.

46B85 ; 20F65 ; 47H10

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- 376 p.
ISBN 978-0-582-07250-3

Pitman monographs and surveys in pure and applied mathematics , 0064

Localisation : Ouvrage RdC (DEVI)

espace de Banach # espace linéaire normé # géométrie et structure des espaces linéaires normés # lissage

46B09 ; 46B10 ; 46B28 ; 46B40 ; 46Bxx

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- 237 p.
ISBN 978-2-7381-0422-9

Localisation : Ouvrage RdC (GODE)

Archimède # Babylone # Cantor # Copernic # Galilée # Kepler # algèbre et algorithme # axiome de l'arithmétique # axiome de la théorie des ensembles # base de numération # bâton et pierre # géométrie # main # polynôme # quaternion # suite de Fibonacci # théorie des nombres

01A17 ; 01A40 ; 03Exx ; 11-03 ; 85-03

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- 194 p.
ISBN 978-0-8218-3304-9

Mathematical world , 0021

Localisation : Collection 1er étage

histoire des nombres # histoire des mathématiques # suite de Fibonacci # théorie des nombres # algorithme # quaternion # géométrie

01A05 ; 11-03 ; 11-01 ; 51-01

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- 360 p.
ISBN 978-2-84225-058-4

Le sel et le fer , 0012

Localisation : Ouvrage RdC (LECO)

histoire des mathématiques # théorie de la démonstration # programme de Hilbert # logique linéaire # nombre entier # cryptologie # fonction modulaire # transcendance # ensemble rectifiable # controlabilité # topologie # forme différentielle # nombre p-adique # représentation de Galois # équation diophantienne # courbe algébrique # groupe fondamental # marche aléatoire

00B15 ; 00A05 ; 01A60

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- 238 p.
ISBN 978-2-7381-2322-0

Sciences

Localisation : Loisir RdC

philosophie des mathématiques # histoire des mathématiques # figure # logique # théorème

01Axx

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- xv; 508 p.
ISBN 978-1-107-65034-3

Localisation : Ouvrage RdC (CHOI)

théorème de Tauberian # théorème des nombres premiers # fonction monotone nulle part dérivable # paradoxe de Banach-Tarski # fonction thêta # somme de caractères # somme exponentielle # conjecture de Littlewood # couronne # algèbre de Banach # sous-espaces complémentaires

00A05 ; 01A60 ; 11T23 ; 11T24 ; 26A27 ; 30H05 ; 33E99 ; 40E05 ; 42A05 ; 42C20 ; 46B09 ; 46B25 ; 46J15 ; 46J20 ; 54E52

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