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  Poisson solver
   a Green's function based spectral solver for the Poisson equation for 
   unbounded and/or periodic domains

   Written by Mads Mølholm Hejlesen (2018-2019)

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  INTRODUCTION
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  The Poisson solver library developed for solving the Poisson equation 
  lap(A) = -B for unbounded and/or periodic domains with spectral accuracy.
  The Poisson solver is based regularised Green's function solutions which 
  is efficiently computed by an FFT methodology to perform the convolutions.

  The library is distributed under a GNU General Public License by which the 
  terms of usage is specified in the "LICENSE.txt" file included in the main 
  directory.


  INSTALLATION
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  The installation of the library is done by a simple make file in the main 
  directory. If the make command does not work directly you may have to manually
  edit the "makefile" file.
  The only installation requirements at this point is OpenMPI.

  NB. The library is developed on a LINUX platform and problems may arise if 
      used on another platform.


  LANGUAGES
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  The directory includes a C++ and a Fortran 90 version of the Poisson solver 
  library. 
  Future plans may include interfaces to other languages such as Python.


  TESTS AND VALIDATION
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  In the "test" directory a number of validating tests are included which for 
  now also serves as the main tutorial .


  EXAMPLES AND TUTORIALS
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  In the "examples" directory a number of examples on applications are being 
  included.


  LITERATURE
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  [1] M. M. Hejlesen, G. Winckelmans and J. H. Walther.
      Non-singular Green's functions for the unbounded Poisson equation in one, 
      two and three dimensions.
      Applied Mathematics Letters, 89:28-34, 2019.
  [2] M. M. Hejlesen, J. T. Rasmussen, P. Chatelain, and J. H. Walther. 
      A high order solver for the unbounded Poisson equation. 
      J. Comput. Phys., 252:458-467, 2013.
  [3] H. J. Spietz, M. M. Hejlesen and J. H. Walther.
      A regularization method for solving the Poisson equation for mixed 
      unbounded-periodic domains. 
      J. Comput. Phys., 356:439-447, 2018.