1. R. Yordanov. About some spectral properties of the Schr\"odinger equation with energy-dependent potentials generating fully integrable Hamiltonian systems (in Russian), Ann. Univ. Sofia "Kliment Ohridski", Fac. Math. Mec., Livre 2 (Mec.) 78 (1984) 229-252.
2. T. Boiadjiev, R. Yordanov. Microcomputer visualization of the solution of the two-bodies problem and the chase problem in classical mechanics (in Bulgarian), Ann. Univ. Sofia "Kliment Ohridski", Fac. Math. Mec., Livre 2 (Mec.) 78 (1984) 253-266.
3. J. Mishev, R. Yordanov. Microcomputer program for visualization of the motion of a rigid body with a fixed point (in Bulgarian), Ann. Univ. Sofia "Kliment Ohridski", Fac. Math. Mec., Livre 2 (Mec.) 79 (1985) 209-224.
4. S. P. Radev, R. G. Yordanov. Nonlinear instability of a capillary jet surrounded by a coaxial immiscible layer (P5-86-748) (in Russian), preprint JINR, Dubna 1986.
5. R. G. Yordanov. Asymptotics for \lambda\to\infty of the Jost solutions of Schr\"odinger equation with a polynomial dependence of the potential on the spectral parameter \lambda (in Russian), Conference on Differential Equations and Applications, Varna (1986) 186-189.
6. R. G. Yordanov. On the spectral theory of \Lambda-operators generated by Schr\"odinger equations with energy-dependent potentials, DAN Bolg. 40, (11), 1987.
7. R. G. Yordanov. Quasi-Lagrangian character of some soliton equations, DAN Bolg. 41, (3), 1988.
8. R. G. Yordanov. About the inverse scattering problem for a quadratic bundle of Schr\"odinger operators, DAN Bolg. 42, (4), 1989.
9. E. Kh. Khristov, R. G. Yordanov. On the linearized nonlinear evolution equations associated with Zakharov-Shabat system, DAN Bolg. 43, (12), 1990.
10. E. Kh. Khristov, R. G. Yordanov. On Cauchy problem for the linearized version of the generalized nonlinear Schr\"odinger equation, Ann. Univ. Sofia "Kliment Ohridski", Fac. Math. Mec., Livre 2 (Mec.) 80 (1986) 191-204.
11. R. G. Yordanov. Cauchy problem for the linearized version of the generalized polynomial KdV equation, J. Math. Phys. 33 (6) 1992.
12. R. G. Yordanov. Why do soliton equations come in hierarchies? J. Math. Phys. 34 (9) 1993.
13. J. R. Heflin, D. Marciu, S. Wang, C. Figura, R. Yordanov. Wavelength Dependence of Optical Limiting in C_60, C_60 Charge-Transfer Complexes, and Phthalocyanines, Conference on Lasers and Electro-Optics, Baltimore (1995).
14. J. R. Heflin, S. Wang, D. Marciu, C. Figura, R. Yordanov. Optical limiting in C_60, C_60 Charge-Transfer Complexes, and Higher Fullerenes from 532 to 750 nm, SPIE Proceedings "Fullerenes and Photonics", v. 2530 (1995) 176-187.
15. J. R. Heflin, S. Wang, D. Marciu, R. Yordanov, C. Figura. Dispersion of optical limiting in C_60, C_60 Charge-Transfer Complexes, and Higher Fullerenes, Proceedings of the International Symposium on the Science and Technology of Atomically Engineered Materials, (1995) OR-65.
16. J. R. Heflin, C. Figura, D. Marciu, S. Wang, R. Yordanov. Near-Infrared Optical Limiting of C_60 Derivatives and Higher Fullerenes, Conference on Lasers and Electro-Optics, (1996).
17. J. R. Heflin, D. Marciu, S. Wang, C. Figura, R. Yordanov. Long-Wavelength Optical Limiting of C_60, C_60 Charge-Transfer Complexes, C_60 Derivatives, and Higher Fullerenes, Proceedings of the International Symposium on the Science and Technology of Atomically Engineered Materials, (1996) 501.
18. J. R. Heflin, D. Marciu, C. Figura, S. Wang, R. Yordanov. Optical Limiting Over an Extended Spectral Region by Derivatization of C_60, SPIE, v. 3146 (1997) 142.
19. R. G. Yordanov. Dubrovin-Type Equations for Completely Integrable Systems Associated with a Polynomial Pencil, Serdika Math. J. 24 (1998) 225-256.

Note: Parts of refs. 1, 10 and 12 above have also been included in a book written by my ex-advisor E. Khristov et al, "Spectral Methods in Soliton Equations", Pitman Monographs and Surveys in Pure and Applied Mathematics, # 73 (see Secs. 3.10, 3.11 and 3.A there).