On the Change of Natural Frequencies Induced by Small Constraints R. DuffinA. Schild Indiana University Mathematics Journal 1957 text pdf 10.1512/iumj.1957.6.56040 10.1512/iumj.1957.6.56040 en Indiana Univ. Math. J. 6 (1957) 731 - 758 state-of-the-art mathematics http://iumj.org/access/