Problem 1. For each of the following systems, find a bound on |a| such that the origin is exponentially stable.
(1) x·1 = - x1 - 2x2 + a(x1 - x2)||x||2, x·2 = 2x1 - x2 - a(x1 + x2)||x||2
(2) x·1 = ax12 + x2, x·2 = -x1 - x13/(1 + x12) - x2
(3) x·1 = x2, x·2 = - x1 - x2 + a (x1 - x2)