function f=p11_cyl(r,sigma)
% compute the p11 component in 1D cylindrical geometry
   track=sybt1d(r,false,5) ;
   val=0 ;
   for i=1:5
      segment=2*track{1,i}{3}(1) ;
      if sigma ~= 0
         d=segment/sigma-(akin(3,0)-akin(3,sigma*segment))/sigma^2 ;
      else
         d=pi*segment^2/4 ;
      end
      val=val+track{1,i}{1}*d ;
   end
   f=val/(pi*r^2) ;