is a statistical Markov model in which the system being modeled is assumed to be a Markov process with unobserved (hidden) states. An HMM can be presented as the simplest dynamic Bayesian network. The mathematics behind the HMM was developed by L. E. Baum and coworkers.[1][2][3][4][5] It is closely related to an earlier work on the optimal nonlinear filtering problem by Ruslan L. Stratonovich,[6] who was the first to describe the forward-backward procedure.