$$
\newcommand{\half}{\frac{1}{2}}
\newcommand{\tp}{\thinspace .}
\newcommand{\uex}{{u_{\small\mbox{e}}}}
\newcommand{\Aex}{{A_{\small\mbox{e}}}}
\newcommand{\Oof}[1]{\mathcal{O}(#1)}
\newcommand{\dfc}{\alpha}  % diffusion coefficient
\newcommand{\dx}{\, \mathrm{d}x}
\newcommand{\ds}{\, \mathrm{d}s}
$$
    
 
 
 
 
 
Bibliography 
 -   R. Rannacher. 
    Finite element solution of diffusion problems with irregular data,
    Numerische Mathematik,
    43,
    pp. 309-327,
    1984.