program main
      implicit double precision (a-h,o-z)
      pi=acos(0.D0)*2.d0

      write(6,*) ' pi = ',pi

      sum = 0.d0
      xfac=1.d0
      indx=0
      do i=1,1000
       do j=1,100000
        indx=indx+1
        sum=sum+xfac/dble(indx)/dble(indx)
C pourquoi foire sum=sum+xfac/dble(indx*indx) avec indx > 65500 ?
       end do
       write(6,9901) indx,sqrt(6.D0*sum),pi-sqrt(6.D0*sum)
 9901  format(' I= ',I9,' pi = ',F17.12,'; diff to real pi :',E12.4)
      end do
     
      end