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Linear Algebra, Theory And Applications, 2012a

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10.5. THE JORDAN CANONICAL FORM 259<br />

m k = 1. The vectors which are in ker (A − λ k I) m k<br />

whicharenotinker(A − λ k I) are called<br />

generalized eigenvectors.<br />

To illustrate the main idea used in proving uniqueness in this theorem, consider the<br />

following two matrices.<br />

⎛<br />

⎞ ⎛<br />

⎞<br />

0 1 0 0 0 1 0 0<br />

⎜ 0 0 1 0<br />

⎟<br />

⎝ 0 0 0 0 ⎠ , ⎜ 0 0 0 0<br />

⎟<br />

⎝ 0 0 0 1 ⎠<br />

0 0 0 0 0 0 0 0<br />

The first has one 3×3 block and the second has two 2 × 2 blocks. Initially both matrices<br />

have rank 2. Now lets raise them to a power 2.<br />

⎛<br />

⎜<br />

⎝<br />

0 1 0 0<br />

0 0 1 0<br />

0 0 0 0<br />

0 0 0 0<br />

⎞<br />

2<br />

⎟<br />

⎠<br />

=<br />

⎛<br />

⎜<br />

⎝<br />

0 0 1 0<br />

0 0 0 0<br />

0 0 0 0<br />

0 0 0 0<br />

⎞<br />

⎟<br />

⎠<br />

which has rank 1 and<br />

⎛<br />

⎜<br />

⎝<br />

0 1 0 0<br />

0 0 0 0<br />

0 0 0 1<br />

0 0 0 0<br />

⎞<br />

2<br />

⎟<br />

⎠<br />

=<br />

⎛<br />

⎜<br />

⎝<br />

0 0 0 0<br />

0 0 0 0<br />

0 0 0 0<br />

0 0 0 0<br />

⎞<br />

⎟<br />

⎠<br />

which has no rank. You see, discrepancies occur in the rank upon raising to higher powers<br />

if the blocks are not the same. Now with this preparation, here is the main theorem.<br />

Theorem 10.5.2 Let V be an n dimensional vector space with field of scalars C or some<br />

other field such that the minimal polynomial of A ∈L(V,V ) completely factors into powers<br />

of linear factors. Then there exists a unique Jordan canonical form for A as described in<br />

(10.5) - (10.7), where uniqueness is in the sense that any two have the same number and<br />

size of Jordan blocks.<br />

Proof: It only remains to verify uniqueness. Suppose there are two, J and J ′ . Then these<br />

are matrices of A with respect to possibly different bases and so they are similar. Therefore,<br />

they have the same minimal polynomials and the generalized eigenspaces have the same<br />

dimension. Thus the size of the matrices J (λ k )andJ ′ (λ k ) defined by the dimension of<br />

these generalized eigenspaces, also corresponding to the algebraic multiplicity of λ k ,must<br />

be the same. Therefore, they comprise the same set of positive integers. Thus listing the<br />

eigenvalues in the same order, corresponding blocks J (λ k ) ,J ′ (λ k )arethesamesize.<br />

It remains to show that J (λ k )andJ ′ (λ k ) are not just the same size but also are the<br />

same up to order of the Jordan blocks running down their respective diagonals. It is only<br />

necessary to worry about the number and size of the Jordan blocks making up J (λ k )and<br />

J ′ (λ k ) . Since J, J ′ are similar, so are J −λ k I and J ′ −λ k I. Thus the following two matrices<br />

are similar<br />

⎛<br />

A ≡<br />

⎜<br />

⎝<br />

J (λ 1 ) − λ k I 0<br />

. ..<br />

J (λ k ) − λ k I<br />

. ..<br />

0 J (λ r ) − λ k I<br />

⎞<br />

⎟<br />

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