function f=cij_f(tau0,sigi,sigj,segmenti,segmentj)
      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
   function f=di_f(sig,segment)
      if sig ~= 0
         f=segment/sig-(akin(3,0)-akin(3,sig*segment))/sig^2 ;
      else
         f=pi*segment^2/4 ;
      end