In this paper we give a financial justification, based on non arbitrage conditions,of the (H) hypothesis in default time modelling. We also show how the (H) hypothesis isaffected by an equivalent change of probability measure.[...]

Filtrations have been introduced by Doob and have been a fundamentalfeature of the theory of stochastic processes. Most basic objects, such asmartingales, semimartingales, stopping times or Markov processes involvethe notion of filtration.[...]

In this paper, we provide a solution to two problems which have been open in default time modeling in credit risk. We first show that if $\tau$ is an arbitrary random (default) time such that its Az\'ema's supermartingale $Z_t^\tau=\P(\taut|\F_t)$ is continuous, then $\tau$ avoids stopping...

Copulas are a general tool to construct multivariate distributions and to investigate dependence structure between random variables. However, the concept of copula is not popular in Finance. In this paper, we show that copulas can be extensively used to solve many financial problems.