Geodesic
Integrable boundary-value problems and nonlinear Fourier harmonics
Zapiski Nauchnykh Seminarov POMI, Questions of quantum field theory and statistical physics. Part 11, Tome 199 (1992), pp. 43-50 
R. F. Bikbaev. Integrable boundary-value problems and nonlinear Fourier harmonics. Zapiski Nauchnykh Seminarov POMI, Questions of quantum field theory and statistical physics. Part 11, Tome 199 (1992), pp. 43-50. http://geodesic.mathdoc.fr/item/ZNSL_1992_199_a3/
@article{ZNSL_1992_199_a3,
     author = {R. F. Bikbaev},
     title = {Integrable boundary-value problems and nonlinear {Fourier} harmonics},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {43--50},
     year = {1992},
     volume = {199},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1992_199_a3/}
}
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An integrable boundary-value problem on a segment for $NS$ model is considered. A concept of nonlinear $\theta$-harmonics for this problem is proposed. An exact solution of the integrable problem on the semiaxis is given with the help of reduction to whole axis case.