1. Estimation of the fundamental frequency of beams and plates with varying thickness
2. Rayleigh, Baron, The Theory of Sound, Vol.I, 2nd Ed.Dover, New York, 1945, pp.113-115.
3. Courant, R. and Hilbert, D., Methods of Mathematical Phyics, Vol.I, (1st English Ed.),Wiley Interscience, New York,1953, pp.343-350.