function xi1ox(xsq)
c     I1(kr)/kr
      real y,p1,t,s,p
      y=.25*xsq
      p1=2.
      t=.5*y
      s=1.
   10 continue
      s=s+t
      if(t.lt.0.001*s)go to 20
      p=p1
      p1=p+1.
      t=t*y/(p*p1)
      go to 10
   20 continue
      xi1ox=.5*s
      return
      end
c++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++*