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v2009.01.01 - Convex Optimization

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A.4. SCHUR COMPLEMENT 559<br />

A.4.0.0.2 Example. Sparse Schur conditions.<br />

Setting matrix A to the identity simplifies the Schur conditions. One<br />

consequence relates the definiteness of three quantities:<br />

[ ]<br />

[ ]<br />

I B<br />

I 0<br />

B T ≽ 0 ⇔ C − B T B ≽ 0 ⇔<br />

C<br />

0 T C −B T ≽ 0 (1422)<br />

B<br />

<br />

A.4.0.0.3 Exercise. Eigenvalues λ of sparse Schur-form.<br />

Prove: given C −B T B = 0, for B ∈ R m×n and C ∈ S n<br />

⎧<br />

([ ]) ⎪⎨ 1 + λ(C) i , 1 ≤ i ≤ n<br />

I B<br />

λ<br />

B T = 1, n < i ≤ m<br />

C<br />

i<br />

⎪⎩<br />

0, otherwise<br />

(1423)<br />

<br />

A.4.0.0.4 Theorem. Rank of partitioned matrices.<br />

When symmetric matrix A is invertible and C is symmetric,<br />

[ ] [ ]<br />

A B A 0<br />

rank<br />

B T = rank<br />

C 0 T C −B T A −1 B<br />

= rankA + rank(C −B T A −1 B)<br />

(1424)<br />

equals rank of a block on the main diagonal plus rank of its Schur complement<br />

[344,2.2, prob.7]. Similarly, when symmetric matrix C is invertible and A<br />

is symmetric,<br />

[ ] [ ]<br />

A B A − BC<br />

rank<br />

B T = rank<br />

−1 B T 0<br />

C<br />

0 T C<br />

(1425)<br />

= rank(A − BC −1 B T ) + rankC<br />

Proof. The first assertion (1424) holds if and only if [176,0.4.6(c)]<br />

[ ] [ ]<br />

A B A 0<br />

∃ nonsingular X,Y X<br />

B T Y =<br />

C 0 T C −B T A −1 (1426)<br />

B<br />

Let [176,7.7.6]<br />

Y = X T =<br />

[ I −A −1 B<br />

0 T I<br />

]<br />

⋄<br />

(1427)

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