code: def parabolic(f, x):
      
      xv = 1/2. * (f[x-1] - f[x+1]) / (f[x-1] - 2 * f[x] + f[x+1]) + x
      yv = f[x] - 1/4. * (f[x-1] - f[x+1]) * (xv - x)
      return (xv, yv)
docs: Interpolation. From ageobot, from somewhere else.