4) D. J. Broad and H. Wakita : “Piecewise-planar representation of vowel formant frequencies”, J. Acoust. Soc. Am., 62, 6, pp. 1467-1473 (Dec. 1977)
5) S. Sato, M. Yokota and H. Kasuya : “Statistical Relationships among the First Three Formant Frequencies in Vowel Segments in Continuous Speech”, Phonetica, 39, 1, pp. 36-46 (1982)