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X: 20, 422-431, LNM 511 (1976)

**YAN, Jia-An**; **YOEURP, Chantha**

Représentation des martingales comme intégrales stochastiques des processus optionnels (Martingale theory, Stochastic calculus)

An attempt to build a theory similar to the previsible representation property with respect to a basic local martingale, but using the optional stochastic integral instead of the standard one

Comment: Apparently this ``optional representation property'' has not been used since

Keywords: Optional stochastic integrals

Nature: Original

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X: 21, 432-480, LNM 511 (1976)

**YOEURP, Chantha**

Décomposition des martingales locales et formules exponentielles (Martingale theory, Stochastic calculus)

It is shown that local martingales can be decomposed uniquely into three pieces, a continuous part and two purely discontinuous pieces, one with accessible jumps, and one with totally inaccessible jumps. Two beautiful lemmas say that a purely discontinuous local martingale whose jumps are summable is a finite variation process, and if it has accessible jumps, then it is the sum of its jumps without compensation. Conditions are given for the existence of the angle bracket of two local martingales which are not locally square integrable. Lemma 2.3 is the lemma often quoted as ``Yoeurp's Lemma'': given a local martingale $M$ and a previsible process of finite variation $A$, $[M,A]$ is a local martingale. The definition of a local martingale on an open interval $[0,T[$ is given when $T$ is previsible, and the behaviour of local martingales under changes of laws (Girsanov's theorem) is studied in a set up where the positive martingale defining the mutual density is replaced by a local martingale. The existence and uniqueness of solutions of the equation $Z_t=1+\int_0^t\tilde Z_s dX_s$, where $X$ is a given special semimartingale of decomposition $M+A$, and $\widetilde Z$ is the previsible projection of the unknown special semimartingale $Z$, is proved under an assumption that the jumps $ėlta A_t$ do not assume the value $1$. Then this ``exponential'' is used to study the multiplicative decomposition of a positive supermartingale in full generality

Comment: The problems in this paper have some relation with Kunita 1005 (in a Markovian set up), and are further studied by Yoeurp in LN**1118**, *Grossissements de filtrations,* 1985. The subject of multiplicative decompositions of positive submartingales is much more difficult since they may vanish. For a simple case see in this volume Yoeurp-Meyer 1023. The general case is due to Azéma (*Z. für W-theorie,* **45,** 1978, presented in 1321) See also 1622

Keywords: Stochastic exponentials, Multiplicative decomposition, Angle bracket, Girsanov's theorem, Föllmer measures

Nature: Original

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X: 23, 501-504, LNM 511 (1976)

**MEYER, Paul-André**; **YOEURP, Chantha**

Sur la décomposition multiplicative des sousmartingales positives (Martingale theory)

This paper expands part of Yoeurp's paper 1021, to cover the decomposition of positive submartingales instead supermartingales, assuming that the process never vanishes. A corollary is that every positive (not necessarily strictly so) submartingale $X_t$ is the optional projection of an increasing process $C_t$, non-adapted, such that $0\leq C_t\leq X_{\infty}$

Comment: See the comments on 1021 for the general case. The latter result is related to Meyer 817. For a related paper, see 1203. Further study in 1620

Keywords: Multiplicative decomposition

Nature: Original

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XIII: 08, 118-125, LNM 721 (1979)

**YOEURP, Chantha**

Sauts additifs et sauts multiplicatifs des semi-martingales (Martingale theory, General theory of processes)

First of all, the jump processes of special semimartingales are characterized (using a result of 1121, 1129 on the jump processes of local martingales). Then a similar problem is solved for multiplicative jumps, a result which includes that of Garcia and al. 1206. A technical lemma characterizes optional processes, bounded in $L^1$, whose previsible projection vanishes

Keywords: Jump processes

Nature: Original

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XIII: 53, 614-619, LNM 721 (1979)

**YOEURP, Chantha**

Solution explicite de l'équation $Z_t=1+\int_0^t |Z_{s-}|\,dX_s$ (Stochastic calculus)

The title describes completely the paper

Keywords: Stochastic differential equations

Nature: Original

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XIV: 28, 249-253, LNM 784 (1980)

**YOEURP, Chantha**

Sur la dérivation des intégrales stochastiques (Stochastic calculus)

The following problem is discussed: under which conditions do ratios of the form $\int_t^{t+h} H_s\,dM_s/(M_{t+h}-M_t)$ converge to $H_t$ as $h\rightarrow 0$? It is shown that positive results due to Isaacson (*Ann. Math. Stat.* **40**, 1979) in the Brownian case fail in more general situations

Comment: See also 1529

Keywords: Stochastic integrals

Nature: Original

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XIV: 29, 254-254, LNM 784 (1980)

**YOEURP, Chantha**

Rectificatif à l'exposé de C.S. Chou (Stochastic calculus)

A mistake in the proof of 1337 is corrected, the result remaining true without additional assumptions

Keywords: Local times, Semimartingales, Jumps

Nature: Correction

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XV: 29, 399-412, LNM 850 (1981)

**YOEURP, Chantha**

Sur la dérivation stochastique au sens de Davis (Stochastic calculus, Brownian motion)

The problem is that of estimating $H_t$ from a knowledge of the process $\int H_sdX_s$, where $X_t$ is a continuous (semi)martingale, as a limit of ratios of the form $\int_t^{t+\alpha} H_sdX_s/(X_{t+\alpha}-X_t)$, or replacing $t+\alpha$ by suitable stopping times

Comment: The problem was suggested and partially solved by Mark H.A. Davis (*Teor. Ver. Prim.*, **20**, 1975, 887--892). See also 1428

Keywords: Stochastic integrals

Nature: Original

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XVI: 20, 234-237, LNM 920 (1982)

**YOEURP, Chantha**

Une décomposition multiplicative de la valeur absolue d'un mouvement brownien (Brownian motion, Stochastic calculus)

A positive submartingale like $X_t=|B_t|$ vanishes too often to be represented as a product of a local martingale and an increasing process. Still, one may look for a kind of additive decomposition of $\log X$, from which the required multiplicative decomposition would follow by taking exponentials. Here the (Ito-Tanaka) additive decomposition of $\log(X\lor\epsilon)$ is studied, as well as its limiting behaviour as $\epsilon\rightarrow0$

Comment: See 1023, 1321

Keywords: Multiplicative decomposition, Change of variable formula, Local times

Nature: Original

Retrieve article from Numdam

Représentation des martingales comme intégrales stochastiques des processus optionnels (Martingale theory, Stochastic calculus)

An attempt to build a theory similar to the previsible representation property with respect to a basic local martingale, but using the optional stochastic integral instead of the standard one

Comment: Apparently this ``optional representation property'' has not been used since

Keywords: Optional stochastic integrals

Nature: Original

Retrieve article from Numdam

X: 21, 432-480, LNM 511 (1976)

Décomposition des martingales locales et formules exponentielles (Martingale theory, Stochastic calculus)

It is shown that local martingales can be decomposed uniquely into three pieces, a continuous part and two purely discontinuous pieces, one with accessible jumps, and one with totally inaccessible jumps. Two beautiful lemmas say that a purely discontinuous local martingale whose jumps are summable is a finite variation process, and if it has accessible jumps, then it is the sum of its jumps without compensation. Conditions are given for the existence of the angle bracket of two local martingales which are not locally square integrable. Lemma 2.3 is the lemma often quoted as ``Yoeurp's Lemma'': given a local martingale $M$ and a previsible process of finite variation $A$, $[M,A]$ is a local martingale. The definition of a local martingale on an open interval $[0,T[$ is given when $T$ is previsible, and the behaviour of local martingales under changes of laws (Girsanov's theorem) is studied in a set up where the positive martingale defining the mutual density is replaced by a local martingale. The existence and uniqueness of solutions of the equation $Z_t=1+\int_0^t\tilde Z_s dX_s$, where $X$ is a given special semimartingale of decomposition $M+A$, and $\widetilde Z$ is the previsible projection of the unknown special semimartingale $Z$, is proved under an assumption that the jumps $ėlta A_t$ do not assume the value $1$. Then this ``exponential'' is used to study the multiplicative decomposition of a positive supermartingale in full generality

Comment: The problems in this paper have some relation with Kunita 1005 (in a Markovian set up), and are further studied by Yoeurp in LN

Keywords: Stochastic exponentials, Multiplicative decomposition, Angle bracket, Girsanov's theorem, Föllmer measures

Nature: Original

Retrieve article from Numdam

X: 23, 501-504, LNM 511 (1976)

Sur la décomposition multiplicative des sousmartingales positives (Martingale theory)

This paper expands part of Yoeurp's paper 1021, to cover the decomposition of positive submartingales instead supermartingales, assuming that the process never vanishes. A corollary is that every positive (not necessarily strictly so) submartingale $X_t$ is the optional projection of an increasing process $C_t$, non-adapted, such that $0\leq C_t\leq X_{\infty}$

Comment: See the comments on 1021 for the general case. The latter result is related to Meyer 817. For a related paper, see 1203. Further study in 1620

Keywords: Multiplicative decomposition

Nature: Original

Retrieve article from Numdam

XIII: 08, 118-125, LNM 721 (1979)

Sauts additifs et sauts multiplicatifs des semi-martingales (Martingale theory, General theory of processes)

First of all, the jump processes of special semimartingales are characterized (using a result of 1121, 1129 on the jump processes of local martingales). Then a similar problem is solved for multiplicative jumps, a result which includes that of Garcia and al. 1206. A technical lemma characterizes optional processes, bounded in $L^1$, whose previsible projection vanishes

Keywords: Jump processes

Nature: Original

Retrieve article from Numdam

XIII: 53, 614-619, LNM 721 (1979)

Solution explicite de l'équation $Z_t=1+\int_0^t |Z_{s-}|\,dX_s$ (Stochastic calculus)

The title describes completely the paper

Keywords: Stochastic differential equations

Nature: Original

Retrieve article from Numdam

XIV: 28, 249-253, LNM 784 (1980)

Sur la dérivation des intégrales stochastiques (Stochastic calculus)

The following problem is discussed: under which conditions do ratios of the form $\int_t^{t+h} H_s\,dM_s/(M_{t+h}-M_t)$ converge to $H_t$ as $h\rightarrow 0$? It is shown that positive results due to Isaacson (

Comment: See also 1529

Keywords: Stochastic integrals

Nature: Original

Retrieve article from Numdam

XIV: 29, 254-254, LNM 784 (1980)

Rectificatif à l'exposé de C.S. Chou (Stochastic calculus)

A mistake in the proof of 1337 is corrected, the result remaining true without additional assumptions

Keywords: Local times, Semimartingales, Jumps

Nature: Correction

Retrieve article from Numdam

XV: 29, 399-412, LNM 850 (1981)

Sur la dérivation stochastique au sens de Davis (Stochastic calculus, Brownian motion)

The problem is that of estimating $H_t$ from a knowledge of the process $\int H_sdX_s$, where $X_t$ is a continuous (semi)martingale, as a limit of ratios of the form $\int_t^{t+\alpha} H_sdX_s/(X_{t+\alpha}-X_t)$, or replacing $t+\alpha$ by suitable stopping times

Comment: The problem was suggested and partially solved by Mark H.A. Davis (

Keywords: Stochastic integrals

Nature: Original

Retrieve article from Numdam

XVI: 20, 234-237, LNM 920 (1982)

Une décomposition multiplicative de la valeur absolue d'un mouvement brownien (Brownian motion, Stochastic calculus)

A positive submartingale like $X_t=|B_t|$ vanishes too often to be represented as a product of a local martingale and an increasing process. Still, one may look for a kind of additive decomposition of $\log X$, from which the required multiplicative decomposition would follow by taking exponentials. Here the (Ito-Tanaka) additive decomposition of $\log(X\lor\epsilon)$ is studied, as well as its limiting behaviour as $\epsilon\rightarrow0$

Comment: See 1023, 1321

Keywords: Multiplicative decomposition, Change of variable formula, Local times

Nature: Original

Retrieve article from Numdam