function f=p11_sphere(r,sigma)
% compute the p11 component in 1D spherical geometry
   track=sybt1d(r,true,5) ;
   val=0 ;
   for i=1:5
      segment=2*track{1,i}{3}(1) ;
      if sigma ~= 0
         d=segment/sigma-(1-exp(-sigma*segment))/sigma^2 ;
      else
         d=segment^2/2 ;
      end
      val=val+track{1,i}{1}*d ;
   end
   f=val/(4*pi*r^3/3) ;