On some universal sigma finite measures and some extensions of Doob's optional stopping theorem

In this paper, we associate, to any submartingale of class $(\mathrm{\Sigma })$, defined on a filtered probability space $(\Omega, $\mathcal{F}$, $\mathbb{P}$, ($\mathcal{F}$t){t \geq 0})$,whichsatisfiessometechnicalconditions,a$\sigma$-finitemeasure$$\mathcal{Q}$$on$(\Omega, $\mathcal{F}$)$, such that for all $t\ge 0$, and for all events $\Lambda_t \in $\mathcal{F}$t$:$$ $\mathcal{Q}$ [\Lambda_t, g\leq t] = $\mathbb{E}${$\mathbb{P}$} [\mathds{1}_{\Lambda_t} X_t]$$where$g$ is the last hitting time of zero of the process $X$. This measure $\mathcal{Q}$ has already been defined in several particular cases, some of them are involved in the study of Brownian penalisation, and others are related with problems in mathematical finance. More precisely, the existence of $\mathcal{Q}$ in the general case solves a problem stated by D. Madan, B. Roynette and M. Yor, in a paper studying the link between Black-Scholes formula and last passage times of certain submartingales. Moreover, the equality defining $\mathcal{Q}$ remains true if one replaces the fixed time $t$ by any bounded stopping time. This generalization can be viewed as an extension of Doob’s optional stopping theorem.