1. I. S. Kac andM. G. Krein:Am. Math. Soc. Trans., II,103, 19 (1974).
2. Some ideas and preliminary results have been presented inAround the string problem, invited lecture in theInternational Meeting on Nonlinear Evolution Equations and Dynamical Systems, June 20–23, 1979, Lecce (Italy).
3. P. C. Sabatier:Geophys. J. R. Astron. Soc,48, 443, 470 (1970).
4. P. C. Sabatier: inInverse and Improperly Posed Problems in Differential Equations, edited byG. Anger (Berlin, 1979).
5. V. Barcilon:Geophys. J. R. Astron. Soc,56, 399 (1979).