Indexing of the full ensemble of physical particles, with the representative particles ordered at the beginning of the sequence. The entirety of particles in a given ensemble is indexed from 1 to N. The first n indices refer to RPs, and the remaining indices belong to non-RPs. Because N may be enormously large, tracking the entirety of particles is not feasible. Only the n RPs are actually modelled on the computer, while the properties of the (N − n) non-RPs are estimated from the known properties of the n representative ones.