a sequence {xn} of real numbers is contractive if ∃ a constant C > 0, 0 < C < 1, such that |xn+1 − xn| ≤ C|xn − xn−1| for all n ∈ N.
$\left|x_{n+1} - x_n\right| < C\left|x_n - x_{n-1}\right|$ prove this is Cauchy