function [ a, c, k ] = fisher_parameters ( a_user, c_user, k_user )

%*****************************************************************************80
%
%% fisher_parameters() returns parameters for the Fisher ODE.
%
%  Discussion:
%
%    If input values are specified, this resets the default parameters.
%    Otherwise, the output will be the current defaults.
%
%  Licensing:
%
%    This code is distributed under the MIT license.
%
%  Modified:
%
%    03 May 2024
%
%  Author:
%
%    John Burkardt
%
%  Input:
%
%    real A_USER: a new value for A.
%
%    real C_USER: a new value for C.
%
%    real K_USER: a new value for K.
%
%  Output:
%
%    real A: the default or new value for A.
%
%    real C: the default or new value for C.
%
%    real K: the default or new value for K.
%
  persistent a_default;
  persistent c_default;
  persistent k_default;
%
%  Initialize defaults.
%
  if ( isempty ( a_default ) )
    a_default = 2.0;
  end

  if ( isempty ( c_default ) )
    c_default = 5.0 / sqrt ( 6.0 );
  end

  if ( isempty ( k_default ) )
    k_default = 1.0 / sqrt ( 6.0 );
  end
%
%  Update defaults if input was supplied.
%
  if ( 1 <= nargin )
    a_default = a_user;
  end

  if ( 2 <= nargin )
    c_default = c_user;
  end

  if ( 3 <= nargin )
    k_default = k_user;
  end
%
%  Return values.
%
  a = a_default;
  c = c_default;
  k = k_default;

  return
end

