function pdf = log_normal_pdf ( x, mu, sigma )

%*****************************************************************************80
%
%% LOG_NORMAL_PDF evaluates the Log Normal PDF.
%
%  Discussion:
%
%    PDF(X)(A,B)
%      = EXP ( - 0.5 * ( ( LOG ( X ) - MU ) / SIGMA )^2 )
%        / ( SIGMA * X * SQRT ( 2 * PI ) )
%
%    The Log Normal PDF is also known as the Cobb-Douglas PDF,
%    and as the Antilog_normal PDF.
%
%    The Log Normal PDF describes a variable X whose logarithm
%    is normally distributed.
%
%    The special case A = 0, B = 1 is known as Gilbrat's PDF.
%
%  Licensing:
%
%    This code is distributed under the GNU LGPL license.
%
%  Modified:
%
%    10 February 1999
%
%  Author:
%
%    John Burkardt
%
%  Parameters:
%
%    Input, real X, the argument of the PDF.
%    0.0 < X
%
%    Input, real MU, SIGMA, the parameters of the PDF.
%    0.0 < SIGMA.
%
%    Output, real PDF, the value of the PDF.
%
  if ( x <= 0.0 )
    pdf = 0.0;
  else
    pdf = exp ( - 0.5 * ( ( log ( x ) - mu ) / sigma ) ^ 2 ) ...
      / ( sigma * x * sqrt ( 2.0 * pi ) );
  end

  return
end
