c
c Time-stamp: <2000-06-15 14:10:58 baron>      
c
      program solve_quad
      implicit none
      real*8 :: x,y,a,b,c,d,e
      real*8 :: dydx,deltax
      real*8, parameter :: tol1=1.d-6,tol2=1.d-3,eps=0.05d0
      integer :: icount
      integer, parameter :: imax=500
c
c  get input values
c      
c sample equation, 1 root 
c give a,b,c,d,e
c   -5 5 .1 2 -10 
c  give initial guess
c  5
c  y =  -0.668071594918551170E-03  x =  1.26559757663514860
c
c      
      print *,"y = a*x**4 + b*x**3 + c*x**2 + d*x + e = 0"
c
      print *,"give a,b,c,d,e"
      read(*,*)a,b,c,d
      print *," give initial guess"
      read(*,*)x
c
c
      icount = 0
      newton_loop: do
      icount = icount + 1
      y = a*x**4 + b*x**3 + c*x**2 + d*x + e
      dydx = 4*a*x**3 + 3*b*x**2 + 2*c*x + d
      deltax = -y/dydx
      deltax = sign(min(abs(deltax),abs(eps*x)),deltax)
c
c check convergence
c      
      if(abs(y) .le. tol1) exit newton_loop
      if(abs(deltax/x) .le. tol2) exit newton_loop
c      
      x = x + deltax
      if(icount .gt. imax) exit newton_loop
      end do newton_loop
      if(icount .gt. imax) then
       print *,"failed to converge in ",imax," iterations"
      endif
      print *,"y = ",y," x = ",x
      end program solve_quad