Noise in the current through a membrane

The electric current through a membrane has been modelled as a transport process of single charged particles through a pore. In this way the average current and the spectral density of the current fluctuations were computed by Barkai et al. in terms of the behavior of the separate particles. I here present another calculation, which is very different from theirs, and to my mind simpler and more direct. For the special example of a Markovian behavior an alternative method is available, which is used here as a check on the general result.