at x1 = -2:        u1        =   x1^3 = -8
at x2 = -1: -u1 + 2u2 - u3   = -6x2   =  6
at x3 =  0: -u2 + 2u3 - u4   = -6x3   =  0
at x4 = +1: -u3 + 2u4 - u5   = -6x4   = -6
at x5 = +2:        u5        =   x5^3 =  8