Geodesic
The Cauchy problem for a semilinear wave equation. III
Zapiski Nauchnykh Seminarov POMI, Differential geometry, Lie groups and mechanics. Part 11, Tome 181 (1990), pp. 24-64 
L. V. Kapitanskii. The Cauchy problem for a semilinear wave equation. III. Zapiski Nauchnykh Seminarov POMI, Differential geometry, Lie groups and mechanics. Part 11, Tome 181 (1990), pp. 24-64. http://geodesic.mathdoc.fr/item/ZNSL_1990_181_a1/
@article{ZNSL_1990_181_a1,
     author = {L. V. Kapitanskii},
     title = {The {Cauchy} problem for a semilinear wave {equation.~III}},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {24--64},
     year = {1990},
     volume = {181},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1990_181_a1/}
}
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Voir la notice du chapitre de livre provenant de la source Math-Net.Ru

The Cauchy problem for a semilinear pseudodifferential second order hyperbolic equation of the form $$ \frac{\partial^2}{\partial t^2}u(t,x)+iB(t)\frac\partial{\partial t}u(t,x)+A(t)u(t,x)+f(t,x;u(t,x))=0 $$ is studied. The results (presented in a previous author's paper, see Zapisky Nauch. Semin. LOMI, 1990, v. 182) on the existence and uniqueness of the global weak (energy class) solutions are revised. In the case of more regular initial data ($u(0,\cdot)\in H^{s+1}$, $\partial_t u(0,\cdot)\in H^s$, $0) the respective regularity of weak solutions is proved.