an:04111196 Zbl 0678.35063 Gushchin, A. K.; Mikhajlov, V. P. Comparison theorems for solutions of hyperbolic equations EN Math. USSR, Sb. 62, No. 2, 349-371 (1989); translation from Mat. Sb., Nov. Ser. 134(176), No. 3(11), 353-374 (1987). 00181391 1989
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35L20 35B40 35D05 quasi-asymptotics; initial-boundary value; integral mean; comparison theorems; existence; Banach space At first, the authors complain why it is useful to introduce a quasi- asymptotics for studying the behaviour of solutions of hyperbolic initial-boundary value problem as $$t\to \infty$$. A function h(t,x) has uniform quasi-asymptotics $$\omega$$ (x) if, roughly speaking its integral mean in t of m-th order of a certain type has the limit $$\omega$$ (x) as $$t\to \infty$$ uniformly in $$x\in \Omega$$. Then the authors consider the following boundary value problem $(*)\quad p(x)u_{tt}- \sum^{n}_{i,j=1}(a_{ij}(x)u_{x_ i})_{x_ j}=f(t,x),\quad \frac{\partial u}{\partial N}|_{\partial \Omega}=0,\quad u|_{t=0}=u_ t|_{t=0}=0,$ with bounded coefficients p, $$a_{ij}$$ and $$p(x)\geq p,=const>0$$. They establish so-called comparison theorems connecting the existence of the quasi-asymptotics to (*) with its existence for the problem (*) with $$p=1$$; the last was studied earlier [see the authors, Mat. Sb. Nov. Ser. 131(173), No.4(12), 419-437 (1986; Zbl 0635.35056)]. So they prove theorems on necessary and sufficient conditions under which there exists such a quasi-asymptotics. The case of $$n=1$$ is separately considered. To establish the theorems the authors study an initial problem in a Banach space, that covers the above problem, and establish corresponding theorems for it. L.Lebedev Zbl 0635.35056