function pdf = zipf_pdf ( x, a )
%*****************************************************************************80
%
%% ZIPF_PDF evaluates the Zipf PDF.
%
% Discussion:
%
% PDF(X)(A) = ( 1 / X^A ) / C
%
% where the normalizing constant is chosen so that
%
% C = Sum ( 1 <= I < oo ) 1 / I^A.
%
% From observation, the frequency of different words in long
% sequences of text seems to follow the Zipf PDF, with
% parameter A slightly greater than 1. The Zipf PDF is sometimes
% known as the "discrete Pareto" PDF.
%
% Lotka's law is a version of the Zipf PDF in which A is 2 or approximately
% 2. Lotka's law describes the frequency of publications by authors in a
% given field, and estimates that the number of authors with X papers is
% about 1/X^A of the number of authors with 1 paper.
%
% Licensing:
%
% This code is distributed under the GNU LGPL license.
%
% Modified:
%
% 08 October 2004
%
% Author:
%
% John Burkardt
%
% Reference:
%
% Alfred Lotka,
% The frequency distribution of scientific productivity,
% Journal of the Washington Academy of Sciences,
% Volume 16, Number 12, 1926, pages 317-324.
%
% Parameters:
%
% Input, integer X, the argument of the PDF.
% 1 <= N
%
% Input, real A, the parameter of the PDF.
% 1.0 < A.
%
% Output, real PDF, the value of the PDF.
%
if ( x < 1 )
pdf = 0.0;
else
c = zeta ( a );
pdf = ( 1.0 / x^a ) / c;
end
return
end