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Stability and asymptotic behaviour of a two-dimensional differential system with delay. (English) Zbl 1008.34064
The two-dimensional system $x'(t)=A(t)x(t)+B(t)x(t-r)+h(t,x(t),x(t-r))$ is considered, where $$A(t)=(a_{jk}(t))$$, $$B(t)=(b_{jk}(t))$$, $$j,k=1,2$$, are real matrices and $$h(t,x,y)$$ is a two-dimensional real vector function. It is supposed that the functions $$a_{jk}$$ are absolutely continuous on $$[t_0,\infty)$$, $$b_{jk}$$ are locally Lebesgue integrable on $$[t_0,\infty)$$ and the function $$h$$ satisfies Carathéodory conditions on $[t_0,\infty)\times\{[x_1,x_2]\in \mathbb{R}^2: x_1^2+x_2^2<K\}\times \{[y_1,y_2]\in \mathbb{R}^2: x_1^2+x_2^2<K\},$ with $$0<K\leq\infty$$. The authors use an original approach for the investigation – with the aid of complex variables the system is rewritten into an equivalent equation with complex-valued coefficients. (This idea was used in a previous paper of the first author, too, see also M. Ráb and J. Kalas [Differ. Integral Equ. 3, No. 1, 127-144 (1990; Zbl 0724.34060)].) Stability and asymptotic stability of the trivial solution, and further asymptotic properties (e.g., the boundedness of all solutions by exponential functions) are studied by means of an appropriate Lyapunov-Krasovskii functional. This approach does not require the uniform stability or uniform asymptotics stability of a corresponding linear system and leads to new, effective and easy applicable results. An illustrative example is considered. The authors discuss possible generalizations, too.
Reviewer: J.Diblík (Brno)

##### MSC:
 34K20 Stability theory of functional-differential equations 34K25 Asymptotic theory of functional-differential equations 34K12 Growth, boundedness, comparison of solutions to functional-differential equations
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