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15 matches found
IV: 15, 170-194, LNM 124 (1970)
MOKOBODZKI, Gabriel
Densité relative de deux potentiels comparables (Potential theory)
The main problem considered here is the following: given a transient resolvent $(V_{\lambda})$ on a measurable space, a finite potential $Vg$, an excessive function $u$ dominated by $Vg$ in the strong sense (i.e., $Vg-u$ is excessive), show that $u=Vf$ for some $f\leq g$, and compute $f$ by some derivation'' procedure, like $\lim_{\lambda\rightarrow\infty} \lambda(I-\lambda V_{\lambda})\,u$
Comment: The main theorem and the technical tools of its proof have been landmarks in the potential theory of a resolvent, though in the case of the resolvent of a good Markov process there is a simple probabilistic proof of the main result. Another exposition can be found in Séminaire Bourbaki, 422, November 1972. See also Chapter XII of Dellacherie-Meyer, Probabilités et potentiel, containing new proofs due to Feyel
Keywords: Resolvents, Strong ordering, Lebesgue derivation theorem
Nature: Original
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IV: 16, 195-207, LNM 124 (1970)
MOKOBODZKI, Gabriel
Quelques propriétés remarquables des opérateurs presque positifs (Potential theory)
A sequel to the preceding paper 415. Almost positive operators are candidates to the role of derivation operators relative to a resolvent
Comment: Same as 415
Keywords: Resolvents, Strong ordering, Lebesgue derivation theorem
Nature: Original
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VI: 17, 173-176, LNM 258 (1972)
MOKOBODZKI, Gabriel
Pseudo-quotient de deux mesures par rapport à un cône de potentiels. Application à la dualité (Potential theory)
The last four pages of this paper have been omitted by mistake, and appear in the following volume as 729. The general results concerning the axiomatically defined cones of potentials (see for instance the author's exposition in Séminaire Bourbaki, 1969-70, 377) are quickly reviewed first, and then applied to the following problem concerning the potential kernel $V$ of a resolvent: given a pair of measures $\lambda\le\mu$ in the sense of balayage, then we have $\lambda V\le \mu V$ in the ordinary sense. The corresponding density (dominated by $1$) does not depend on the resolvent, but only on the potential cones of excessive functions and potentials associated with it, and a way to compute it is indicated
Keywords: Cones of potentials
Nature: Original
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VII: 29, 319-321, LNM 321 (1973)
MOKOBODZKI, Gabriel
Pseudo-quotient de deux mesures, application à la dualité (Potential theory)
Contains the four last pages of 617 omitted from Volume VI
Nature: Correction
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IX: 22, 437-442, LNM 465 (1975)
MOKOBODZKI, Gabriel
Relèvement borélien compatible avec une classe d'ensembles négligeables. Application à la désintégration des mesures (Measure theory)
This is a beautiful application of the continuum hypothesis'' (axiom). It is shown that if $(E, {\cal E})$ is a separable measurable space (or more generally ${\cal E}$ has the power of the continuum) and ${\cal N}$ is a family of negligible sets within ${\cal E}$, then the space of classes of bounded measurable functions mod.~${\cal N}$ has a linear, isometric, and multiplicative lifting. The proof is rather simple. Essentially the same theorem was discovered independently by Chatterji, see Vector and Operator Valued Measures, Academic Press 1973
Comment: The same proof leads to a slightly stronger and useful result (Meyer 2711): if $E$ is a compact metric space and if any two continuous functions equal a.e. are equal everywhere, the lifting can be taken to be the identity on continuous functions, and to be local, i.e., the liftings of two Borel functions equal a.e. in an open set are equal everywhere in this set
Keywords: Continuum axiom, Lifting theorems, Negligible sets
Nature: Original
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X: 28, 540-543, LNM 511 (1976)
MOKOBODZKI, Gabriel
Démonstration élémentaire d'un théorème de Novikov (Descriptive set theory)
Novikov's theorem asserts that any sequence of analytic subsets of a compact metric space with empty intersection can be enclosed in a sequence of Borel sets with empty intersection. This result has important consequences in descriptive set theory (see Dellacherie 915). A fairly simple proof of this theorem is given, which relates it to the first separation theorem (rather than the second separation theorem as it used to be)
Comment: Dellacherie in this volume (1032) further simplifies the proof. For a presentation in book form, see Dellacherie-Meyer, Probabilités et Potentiel C, chapter XI 9
Keywords: Analytic sets
Nature: Original
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XII: 36, 489-490, LNM 649 (1978)
MOKOBODZKI, Gabriel
Domination d'une mesure par une capacité (Measure theory)
A bounded measure $\mu$ is said to be dominated by a capacity $C$ (countably subadditive, continuous along increasing sequences; neither strong subadditivity nor decreasing sequences are mentioned) if all sets of capacity $0$ have also measure $0$. The main result then states that the space can be decomposed into a set $A_0$ of capacity $0$, and disjoint sets $A_n$ on each of which $\mu$ is smaller than a multiple of $C$
Nature: Original
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XII: 37, 491-508, LNM 649 (1978)
MOKOBODZKI, Gabriel
Ensembles à coupes dénombrables et capacités dominées par une mesure (Measure theory, General theory of processes)
Let $X$ be a compact metric space $\mu$ be a bounded measure. Let $F$ be a given Borel set in $X\times R_+$. For $A\subset X$ define $C(A)$ as the outer measure of the projection on $X$ of $F\cap(A\timesR_+)$. Then it is proved that, if there is some measure $\lambda$ such that $\lambda$-null sets are $C$-null (the relation goes the reverse way from the preceding paper 1236!) then $F$ has ($\mu$-a.s.) countable sections, and if the property is strengthened to an $\epsilon-\delta$ absolute continuity'' relation, then $F$ has ($\mu$-a.s.) finite sections
Comment: This was a long-standing conjecture of Dellacherie (707), suggested by the theory of semi-polar sets. For further development see 1602
Keywords: Sets with countable sections
Nature: Original
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XII: 43, 564-566, LNM 649 (1978)
DELLACHERIE, Claude; MOKOBODZKI, Gabriel
Deux propriétés des ensembles minces (abstraits) (Descriptive set theory)
Given a class ${\cal S}$ of Borel sets understood as small'' sets, the class ${\cal L}$ consisting of their conplements understood as large'' sets, a set $A$ is said to be ${\cal S}$-thin if does not contain uncountably many disjoint large'' sets. For instance, if ${\cal S}$ is the class of polar sets, then thin sets are the same as semi-polar sets. Two general theorems are proved here on thin sets
Keywords: Thin sets, Semi-polar sets, Essential suprema
Nature: Original
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XVI: 02, 8-28, LNM 920 (1982)
DELLACHERIE, Claude; FEYEL, Denis; MOKOBODZKI, Gabriel
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XXIII: 26, 324-325, LNM 1372 (1989)
MOKOBODZKI, Gabriel
Opérateur carré du champ : un contre-exemple
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XXV: 19, 220-233, LNM 1485 (1991)
ÉMERY, Michel; MOKOBODZKI, Gabriel
Sur le barycentre d'une probabilité dans une variété (Stochastic differential geometry)
In a manifold $V$ (endowed with a connection), convex functions and continuous martingales can be defined, but expectations cannot. This article proposes to define the mass-centre of a probability $\mu$ on $V$ as a whole set of points, consisting of all $x$ in $V$ such that $f(x)\le\mu(f)$ for all bounded, convex $f$ on $V$. If $V$ is small enough, it is shown that this is equivalent to demanding that there exists (on a suitable filtered probability space) a continuous martingale $X$ such that $X_0=x$ and $X_1$ has law $\mu$
Comment: The conjecture (due to Émery) at the bottom of page 232 has been disproved by Kendall (J. London Math. Soc. 46, 1992), as pointed out in 2650
Keywords: Martingales in manifolds, Convex functions
Nature: Original
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XXVI: 50, 633-633, LNM 1526 (1992)
ÉMERY, Michel; MOKOBODZKI, Gabriel
Correction au Séminaire~XXV (Stochastic differential geometry)
Points out that the conjecture (due to Émery) at the bottom of page 232 in 2519 is refuted by Kendall (J. London Math. Soc. 46, 1992)
Keywords: Martingales in manifolds
Nature: Correction
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XXVII: 27, 304-311, LNM 1557 (1993)
MOKOBODZKI, Gabriel
Représentation d'un semigroupe d'opérateurs sur un espace $L^1$ par des noyaux. Remarques sur deux articles de S.E. Kusnetsov
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XXX: 20, 312-343, LNM 1626 (1996)
AZÉMA, Jacques; JEULIN, Thierry; KNIGHT, Frank B.; MOKOBODZKI, Gabriel; YOR, Marc
Sur les processus croissants de type injectif
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