Entropy Semiring Forward-backward Algorithm for HMM Entropy Computation
  The paper presents Entropy Semiring Forwardbackward algorithm (ESRFB) and its application for memory efficient computation of the subsequence constrained entropy and state sequence entropy of a Hidden Markov Model (HMM) when an observation sequence is given. ESRFB is based on forward-backward recursion over the entropy semiring, having the lower memory requirement than the algorithm developed by Mann and MacCallum, with the same time complexity. Furthermore, when it is used with forward pass only, it is applicable for the computation of HMM entropy for a given observation sequence, with the same time and memory complexity as the previously developed algorithm by Hernando et al 
Velimir M. Ilic
cs.IT math.IT