Itô versus Stratonovich

A survey is given of the facts and fancies concerning the nonlinear Langevin or Itô equation. Actually, it is merely a pre-equation, which becomes an equation when an interpretation rule is added. The rules of Itô and Stratonovich differ, but both are mathematically consistent and therefore equally admissible conventions. The reason why they seem to lead to physical differences is that the Langevin approach used to arrive at the equation involves a tacit assumption. For systems with external noise this assumption can be justified, and it is then clear that the Stratonovich rule applies. Systems with internal noise, however, can only be properly described by a master equation and the Itô-Stratonovich controversy never enters. Afterward one is free to model the resulting fluctuations either with an Itô or a Stratonovich scheme, but that does not lead to any new information.

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