program bisection
      implicit double precision (a-h, o-z)
c
      double precision f
      data a, b, tol/2.0d0, 3.0d0, 1.0d-06/
c
c     To parakatw programma ypologizei ena diasthma [a,b] mhkous
c     to poly tol to opoio periexei th riza ths exiswshs f(x)=0.
c     To programma ypo8etei oti f(a)f(b) < 0.
c
      h = b - a
      n = 1
c
      fa = f(a)
      fb = f(b)

      do while(h .gt. tol)
         c = (a + b) / 2.0d0
         if (c .eq. a .or. c .eq. b) goto 10

         fc = f(c)
         if (fc .eq. 0.0d0) goto 10
c
c        H enyparxousa synarthsh sign orizetai ws exhs:
c
c                       |  |a1|, an a2 >= 0
c        sign(a1, a2) = |
c                       | -|a1|, an a2 < 0
c
         if (dsign(1.0d0, fb) .ne. dsign(1.0d0, fc)) then
            a = c
            fa = fc
         else
            b = c
            fb = fc
         endif

         write(6, 20) n, c, fc, h

         h = h / 2.0d0
         n = n + 1
      enddo

 10   write(6, 30) (a + b)/2.0d0, h
 20   format(1x, i2, 1x, d16.9, 1x, d16.9, 1x, d16.9)
 30   format(/1x, 'Root   :', d16.9, /1x, 'Length :', d16.9)
      stop
      end
c
      double precision function f(x)
      double precision x

      f = -5.0d0 - x * (2.0d0 - x * x)

      return
      end