Stochastic logarithm
In stochastic calculus, stochastic logarithm of a semimartingale such that and is the semimartingale given by[1]
In layperson's terms, stochastic logarithm of measures the cumulative percentage change in .
Notation and terminology
The process obtained above is commonly denoted . The terminology stochastic logarithm arises from the similarity of to the natural logarithm : If is absolutely continuous with respect to time and , then solves, path-by-path, the differential equation
whose solution is .
General formula and special cases
- Without any assumptions on the semimartingale (other than ), one has[1]where is the continuous part of quadratic variation of and the sum extends over the (countably many) jumps of up to time .
- If is continuous, then In particular, if is a geometric Brownian motion, then is a Brownian motion with a constant drift rate.
- If is continuous and of finite variation, thenHere need not be differentiable with respect to time; for example, can equal 1 plus the Cantor function.
Properties
- Stochastic logarithm is an inverse operation to stochastic exponential: If , then . Conversely, if and , then .[1]
- Unlike the natural logarithm , which depends only of the value of at time , the stochastic logarithm depends not only on but on the whole history of in the time interval . For this reason one must write and not .
- Stochastic logarithm of a local martingale that does not vanish together with its left limit is again a local martingale.
- All the formulae and properties above apply also to stochastic logarithm of a complex-valued .
- Stochastic logarithm can be defined also for processes that are absorbed in zero after jumping to zero. Such definition is meaningful up to the first time that reaches continuously.[2]
Useful identities
Applications
- Girsanov's theorem can be paraphrased as follows: Let be a probability measure equivalent to another probability measure . Denote by the uniformly integrable martingale closed by . For a semimartingale the following are equivalent:
- Process is special under .
- Process is special under .
- + If either of these conditions holds, then the -drift of equals the -drift of .
References
- Jacod, Jean; Shiryaev, Albert Nikolaevich (2003). Limit theorems for stochastic processes (2nd ed.). Berlin: Springer. pp. 134–138. ISBN 3-540-43932-3. OCLC 50554399.
- Larsson, Martin; Ruf, Johannes (2019). "Stochastic exponentials and logarithms on stochastic intervals — A survey". Journal of Mathematical Analysis and Applications. 476 (1): 2–12. doi:10.1016/j.jmaa.2018.11.040. S2CID 119148331.
See also
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