function f=cij_f(tau0,sigi,sigj,segmenti,segmentj)
% function f=cij_f(tau0,sigi,sigj,segmenti,segmentj)
% (c) 2006 Alain Hebert, Ecole Polytechnique de Montreal
  if sigi ~= 0 && sigj ~= 0
    f=(akin(3,tau0)-akin(3,tau0+sigi*segmenti)-akin(3,tau0+sigj*segmentj)+ ...
       akin(3,tau0+sigi*segmenti+sigj*segmentj))/(sigi*sigj) ;
  elseif sigi == 0 && sigj ~= 0
    f=(akin(2,tau0)-akin(2,tau0+sigj*segmentj))*segmenti/sigj ;
  elseif sigi ~= 0 && sigj == 0
    f=(akin(2,tau0)-akin(2,tau0+sigi*segmenti))*segmentj/sigi ;
  else
    f=akin(1,tau0)*segmenti*segmentj ;
  end