A boundary value problem for the elliptic equation of second order in a domain with a narrow slit. 1. The two-dimensional case

An elliptic equation is considered in a domain $D_\varepsilon$ that is obtained from a two-dimensional domain $D$ by removing a neighborhood of a segment. It is assumed that this neighborhood contracts to the segment as $\varepsilon\to0$. An asymptotic expansion of the solution of the first boundary value problem in $D_\varepsilon$ is constructed and justified.
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