References 
 -   L. N. Trefethen. 
    Trefethen's index cards - Forty years of notes about People, Words and Mathematics,
    World Scientific,
    2011.
 
 -   H. P. Langtangen. 
    Finite Difference Computing with Exponential Decay Models,
    Springer,
    2015,
    http://tinyurl.com/nclmcng/web.
 
 -   R. Rannacher. 
    Finite element solution of diffusion problems with irregular data,
    Numerische Mathematik,
    43,
    pp. 309-327,
    1984.
 
 -   H. P. Langtangen. 
    Scaling of Differential Equations,
    2015,
    http://tinyurl.com/qfjgxmf/web.