Exercise 3: Predicting Long-Term Behavior From Eigenvalue Magnitudes — Possible Solution ==================================================================== GIVEN ------------------------------ L1 = 0.5 (eigenvector direction u1) L2 = 1.5 (eigenvector direction u2) WHICH DIRECTION DOMINATES ------------------------------ Per this chapter's own repeated-application finding, applying a transformation many times to a generic starting vector drives the result increasingly toward the eigenvector belonging to the LARGEST-MAGNITUDE eigenvalue. Comparing |L1| = 0.5 and |L2| = 1.5, L2 is the larger of the two, so after many repeated applications the result will end up dominated by the u2 direction - any starting contribution along u1 becomes comparatively negligible. WHY u1'S CONTRIBUTION FADES OUT ------------------------------ Each repeated application multiplies the component of the vector lying along u1 by another factor of L1 = 0.5. Repeatedly multiplying by a number less than 1 in magnitude shrinks that component toward zero the more times the transformation is applied - so the u1 part of the vector effectively disappears over many iterations, even though it never becomes exactly zero. WHY u2'S CONTRIBUTION GROWS ------------------------------ The component along u2 gets multiplied by L2 = 1.5 at every step. Repeatedly multiplying by a number greater than 1 in magnitude grows that component larger and larger without bound, exactly the behavior this chapter's own worked example demonstrated with L = 3 (roughly tripling in magnitude at each step). OVERALL MAGNITUDE: GROWS WITHOUT BOUND ------------------------------ Since the dominant eigenvalue's magnitude (1.5) is greater than 1, the overall vector's magnitude grows without bound as the transformation is applied more and more times - this is the unstable case from this chapter's own stability-analysis application, not the stable, shrinking-toward-zero case (which would require every eigenvalue's magnitude to be below 1). WHY THIS WORKS AS AN ANSWER ------------------------------ The prediction is built directly from this chapter's own repeated- application finding (dominance follows the largest-magnitude eigenvalue), explains both the fading and the growing contributions in terms of what repeated multiplication by a number below vs. above 1 actually does, and correctly classifies the overall system as unstable using this chapter's own stability-analysis criterion rather than guessing.