function f=di_f(sig,segment)
% function f=di_f(sig,segment)
% (c) 2006 Alain Hebert, Ecole Polytechnique de Montreal
  if sig ~= 0
    f=segment/sig-(akin(3,0)-akin(3,sig*segment))/sig^2 ;
  else
    f=pi*segment^2/4 ;
  end