Subroutine Newton(x,a,maxit)
  Implicit None
  Real, Parameter :: tol=0.1**15
  Real :: x,a,v
  Integer :: maxit, i
  Real :: ft_f, ft_fp ! f(x), f'(x)

    Write(*,"(I4,A,E23.16,A,pE23.16)") 0,"-th x = ",a," value = ",ft_f(a) 
  Do i=1,maxit
    If (ft_fp(a)==0) Exit
    x = a - ft_f(a)/ft_fp(a)
    a = x
    Write(*,"(I4,A,E23.16,A,pE23.16)") i,"-th x = ",x," value = ",ft_f(x) 
    If (abs(ft_f(x))<tol) Exit
  EndDo
    maxit = i

End Subroutine Newton