Let $\{X_1, X_2,\cdots\}$ be a stationary process with probability densities $f(X_1, X_2,\cdots, X_n)$ with respect to Lebesgue measure or with respect to a Markov measure with a stationary transition measure. It is shown that the sequence of relative entropy densities $(1/n)\log f(X_1, X_2,\cdots, X_n)$ converges almost surely. This long-conjectured result extends the $L^1$ convergence obtained by Moy, Perez, and Kieffer and generalizes the Shannon-McMillan-Breiman theorem to nondiscrete processes. The heart of the proof is a new martingale inequality which shows that logarithms of densities are $L^1$ dominated.
@article{1176992813,
author = {Barron, Andrew R.},
title = {The Strong Ergodic Theorem for Densities: Generalized Shannon-McMillan-Breiman Theorem},
journal = {Ann. Probab.},
volume = {13},
number = {4},
year = {1985},
pages = { 1292-1303},
language = {en},
url = {http://dml.mathdoc.fr/item/1176992813}
}
Barron, Andrew R. The Strong Ergodic Theorem for Densities: Generalized Shannon-McMillan-Breiman Theorem. Ann. Probab., Tome 13 (1985) no. 4, pp. 1292-1303. http://gdmltest.u-ga.fr/item/1176992813/