“…Problems of this kind have been investigated by several authors, in different context, partly under specific assumptions on the geometry or the involved dimensions, cf. Lemrabet [14], Escauriaza, Fabes, and Verchota [6], Torres and Welland [21], Chkadua [3,4], Li and Vogelius [13], Li and Nirenberg [12], Nicaise and Sändig [16] (numerical method), Heinrich, Nicaise and Weber [8] (the Fourier-finite-element method and singular functions of non-tensorial type), Kapanadze and Schulze [10] (the latter paper studies the case with conical singularities at the interfaces).…”