1. An Introduction to Probability Theory and Its Applications, Vol. 1;Feller,1968
2. An Introduction to Probability Theory and Its Applications, Vol. 2;Feller,1971
3. A. Rényi, Proc. 4th Berkeley Symp. Math. Stat. Prob., Vol. 1960, 1961, p. 547.
4. Perplexity—a measure of the difficulty of speech recognition tasks