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Linear Algebra, Theory And Applications, 2012a

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370 NORMS FOR FINITE DIMENSIONAL VECTOR SPACES<br />

25. Using Problem 24 show that if A is a matrix having the real parts of all eigenvalues<br />

less than 0 then if<br />

Ψ ′ (t) =AΨ(t) , Ψ (0) = I<br />

it follows<br />

lim Ψ(t) =0.<br />

t→∞<br />

Hint: Consider the columns of Ψ (t)?<br />

26. Let Ψ (t) be a fundamental matrix satisfying<br />

Ψ ′ (t) =AΨ(t) , Ψ (0) = I.<br />

Show Ψ (t) n =Ψ(nt) . Hint: Subtract and show the difference satisfies Φ ′ = AΦ, Φ (0) =<br />

0. Use uniqueness.<br />

27. If the real parts of the eigenvalues of A are all negative, show that for every positive<br />

t,<br />

lim Ψ(nt) =0.<br />

n→∞<br />

Hint: Pick Re (σ (A)) < −λ

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