1. See R. Courant and D. Hilbert,Methods of Mathematical Physics(Interscience, New York, 1953), Chap. V, Sec. 3 (vibrating strings).
2. The Synthesis of Nonuniform Lines of Finite Length-Part I
3. Continuous and Discrete Inverse‐Scattering Problems in a Stratified Elastic Medium. I. Plane Waves at Normal Incidence
4. See also I. Kay, in “Mathematics of Profile Inversion,” NASA TMX-62, 150 (1972), p. 6.7.
5. A discussion of the effects of positivity constraints appears in P. C. Sabatier, “On geophysical inverse problems and constraints,” Sec. 2, to be published in the Z. Geophys.