English

Bene$\check{\bf S}$ condition for discontinuous exponential martingale

Probability 2009-12-07 v3

Abstract

It is known the Girsanov exponent zt\mathfrak{z}_t, being solution of Doleans-Dade equation zt=1+0tα(ω,s)dBs \mathfrak{z_t}=1+\int_0^t\alpha(\omega,s)dB_s generated by Brownian motion BtB_t and a random process α(ω,t)\alpha(\omega,t) with 0tα2(ω,s)ds<\int_0^t\alpha^2(\omega,s)ds<\infty a.s., is the martingale provided that the Benesˇ{\rm \check{s}} condition α(ω,t)2const.[1+sups[0,t]Bs2],t>0, |\alpha(\omega,t)|^2\le \text{\rm const.}\big[1+\sup_{s\in[0,t]}B^2_s\big], \forall t>0, holds true. In this paper, we show BtB_t can be replaced by by a homogeneous purely discontinuous square integrable martingale MtM_t with independent increments and paths from the Skorokhod space D[0,) \mathbb{D}_{[0,\infty)} having positive jumps Mt\triangle M_t with \Es[0,t](Ms)3<\E\sum_{s\in[0,t]}(\triangle M_s)^3<\infty. A function α(ω,t)\alpha(\omega,t) is assumed to be nonnegative and predictable. Under this setting zt\mathfrak{z}_t is the martingale provided that α2(ω,t)const.[1+sups[0,t]Ms2], t>0. \alpha^2(\omega,t)\le \text{\rm const.}\big[1+\sup_{s\in[0,t]}M^2_{s-}\big], \ \forall t>0. The method of proof differs from the original Benesˇ{\rm \check{s}} one and is compatible for both setting with BtB_t and MtM_t.

Keywords

Cite

@article{arxiv.0911.0641,
  title  = {Bene$\check{\bf S}$ condition for discontinuous exponential martingale},
  author = {R. Liptser},
  journal= {arXiv preprint arXiv:0911.0641},
  year   = {2009}
}

Comments

8 pages

R2 v1 2026-06-21T14:07:06.063Z