Let X be the unique normal martingale such that X0=0 and d[X]t=(1−t−Xt−) dXt+dt and let Yt:=Xt+t for all t≥0; the semimartingale Y arises in quantum probability, where it is the monotone-independent analogue of the Poisson process. The trajectories of Y are examined and various probabilistic properties are derived; in particular, the level set {t≥0: Yt=1} is shown to be non-empty, compact, perfect and of zero Lebesgue measure. The local times of Y are found to be trivial except for that at level...

Let N ≥ 2 be a given integer. Suppose that $df={\left(dfₙ\right)}_{n\ge 0}$ is a martingale difference sequence with values in $\ell {₁}^N$ and let ${\left(\epsilon ₙ\right)}_{n\ge 0}$ be a deterministic sequence of signs. The paper contains the proof of the estimate $\mathbb{P}(su{p}_{n\ge 0}\left|\right|{\sum }_{k=0}^n{\epsilon }_{k}d{f}_k{\left|\right|}_{\ell {₁}^N}\ge 1)\le (lnN+ln\left(3lnN\right))/(1-{\left(2lnN\right)}^{-1})su{p}_{n\ge 0}\left|\right|{\sum }_{k=0}^{n}d{f}_k{\left|\right|}_{\ell {₁}^N}$. It is shown that this result is asymptotically sharp in the sense that the least constant $C_N$ in the above estimate satisfies $li{m}_{N\to \infty }{C}_N/lnN=1$. The novelty in the proof is the explicit verification of the ζ-convexity of the space $\ell {₁}^N$.

A martingale problem approach is used first to analyze compactness and continuous dependence of the solution set to stochastic differential inclusions of Ito type with convex integrands on the initial distributions. Next the problem of existence of optimal weak solutions to such inclusions and their dependence on the initial distributions is investigated.

Let ${𝔽}_{}^{}$F be a filtration andτbe a random time. Let ${𝔾}_{}^{}$G be the progressive enlargement of ${𝔽}_{}^{}$F withτ. We study the following formula, called the optional splitting formula: For any ${𝔾}_{}^{}$G-optional processY, there exists an ${𝔽}_{}^{}$F-optional processY′ and a function Y′′ defined on [0,∞] × (ℝ+ × Ω) being $ℬ[0,\infty ]\otimes 𝒪\left({𝔽}_{}^{}\right)$ℬ[0,∞]⊗x1d4aa;(F) measurable, such that $Y=Y^{\text{'}}{1}_{[0,\tau )}^{}+Y^{\text{'}\text{'}}\left(\tau \right){\mathbb{1}}_{[\tau ,\infty )}^{}.$Y=Y′1[0,τ)+Y′′(τ)1[τ,∞). (This formula can also be formulated for multiple random timesτ1,...,τk). We are interested in this formula because of its fundamental role in many...

Penalisation involving the one-sided supremum for a stable Lévy process with index α∈(0, 2] is studied. We introduce the analogue of Azéma–Yor martingales for a stable Lévy process and give the law of the overall supremum under the penalised measure.

Dans cet article, nous pénalisons la loi d’une araignée brownienne ${\left(A_t\right)}_{t\ge 0}$ prenant ses valeurs dans un ensemble fini $E$ de demi-droites concourantes, avec un poids égal à $\frac{1}{Z_t}exp({\alpha }_{N_t}{X}_t+\gamma L_t)$, où $t$ est un réel positif, ${\left({\alpha }_k\right)}_{k\in E}$ une famille de réels indexés par $E$, $\gamma$ un paramètre réel, $X_t$ la distance de $A_t$ à l’origine, $N_t$ ($\in E$) la demi-droite sur laquelle se trouve $A_t$, $L_t$ le temps local de ${\left(X_s\right)}_{0\le s\le t}$ à l’origine, et $Z_t$ la constante de normalisation. Nous montrons que la famille des mesures de probabilité obtenue par ces pénalisations converge vers...

As in preceding papers in which we studied the limits of penalized 1-dimensional Wiener measures with certain functionals Γt, we obtain here the existence of the limit, as t → ∞, of d-dimensional Wiener measures penalized by a function of the maximum up to time t of the Brownian winding process (for d = 2), or in {d}≥ 2 dimensions for Brownian motion prevented to exit a cone before time t. Various extensions of these multidimensional penalisations are studied, and the limit laws are described....