词条 | Gamma process |
释义 |
}} A gamma process is a random process with independent gamma distributed increments. Often written as , it is a pure-jump increasing Lévy process with intensity measure for positive . Thus jumps whose size lies in the interval occur as a Poisson process with intensity The parameter controls the rate of jump arrivals and the scaling parameter inversely controls the jump size. It is assumed that the process starts from a value 0 at t=0. The gamma process is sometimes also parameterised in terms of the mean () and variance () of the increase per unit time, which is equivalent to and . PropertiesSince we use the Gamma function in these properties, we may write the process at time as to eliminate ambiguity. Some basic properties of the gamma process are:{{citation needed|date=February 2012}} Marginal distributionThe marginal distribution of a gamma process at time is a gamma distribution with mean and variance That is, its density is given by ScalingMultiplication of a gamma process by a scalar constant is again a gamma process with different mean increase rate. Adding independent processesThe sum of two independent gamma processes is again a gamma process. Momentswhere is the Gamma function. Moment generating functionCorrelation, for any gamma process The gamma process is used as the distribution for random time change in the variance gamma process. References
1 : Lévy processes |
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