function f = p18_fun ( n, p )

%*****************************************************************************80
%
%% p18_fun evaluates the integrand for problem 18.
%
%  Integrand:
%
%    f(x,y) = log ( x + y )
%
%  Vertices:
%
%    (0,0), (1,0), (0,1)
%
%  Integral:
%
%    -1/4
%
%  Licensing:
%
%    This code is distributed under the MIT license.
%
%  Modified:
%
%    10 April 2007
%
%  Author:
%
%    John Burkardt
%
%  Reference:
%
%    Elise deDoncker, Ian Robinson,
%    Algorithm 612:
%    Integration over a Triangle Using Nonlinear Extrapolation,
%    ACM Transactions on Mathematical Software,
%    Volume 10, Number 1, March 1984, pages 17-22.
%
%  Input:
%
%    integer N, the number of evaluation points.
%
%    real P(2,N), the evaluation points.
%
%  Output:
%
%    real F(N), the function values.
%
  f(1:n) = -4.0 * log ( p(1,1:n) + p(2,1:n) );

  return
end
