1. N. W. McLachlan. 'Theory of Vibrations', (Dover, New York, 1951), 86.
2. Lord Rayleigh. 'Theory of Sound', Vol.1,(MacMillan, London, 1894), 250.
3. P. H. Hermans. 'Contributions to the Physics of Cellulose', (Elsevier, Amsterdam, 1946), 56.
4. J. J. Stoker. 'Non-linear Vibrations', (Inters ience, New York, 1950), 83.
5. R. S. Marvin. National Bureau of Standards Report 2305, 1953.