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Importance sampling, Fokker-Planck and Langevin equations

If we assume a discrete set of events, our initial probability distribution function can be given by

w_i(0) = \delta_{i,0},

and its time-development after a given time step \Delta t=\epsilon is

w_i(t) = \sum_{j}W(j\rightarrow i)w_j(t=0).

The continuous analog to w_i(0) is

w(\mathbf{x})\rightarrow \delta(\mathbf{x}),

where we now have generalized the one-dimensional position x to a generic-dimensional vector \mathbf{x} . The Kroenecker \delta function is replaced by the \delta distribution function \delta(\mathbf{x}) at t=0 .