function xi0(xsq)
c    I0(kr)
      real y,p,t,s
      y=.25*xsq
      p=1.
      t=y
      s=1.
   10 continue
      s=s+t
      if(t.lt.0.001*s)go to 20
      p=p+1.
      t=t*y/p**2
      go to 10
   20 continue
      xi0=s
      return
      end
c++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++*