function f=rij_f(sg,tau0,sigi,sigj,segmenti,segmentj)
% function f=rij_f(sg,tau0,sigi,sigj,segmenti,segmentj)
% (c) 2006 Alain Hebert, Ecole Polytechnique de Montreal
   if sigi ~= 0 && sigj ~= 0
      f=0.5*(taben(3,tau0)-taben(3,tau0+sg*sigi*segmenti)- ...
             taben(3,tau0+sg*sigj*segmentj)+ ...
             taben(3,tau0+sg*sigi*segmenti+sg*sigj*segmentj))/(sigi*sigj) ;
   elseif sigi == 0 && sigj ~= 0
      f=sg*0.5*segmenti*(taben(2,tau0)-taben(2,tau0+sg*sigj*segmentj))/sigj ;
   elseif sigi ~= 0 && sigj == 0
      f=sg*0.5*segmentj*(taben(2,tau0)-taben(2,tau0+sg*sigi*segmenti))/sigi ;
   else
      f=0.5*segmenti*segmentj*taben(1,tau0) ;
   end