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v2009.01.01 - Convex Optimization

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2.9. POSITIVE SEMIDEFINITE (PSD) CONE 119<br />

0<br />

-1<br />

-0.5<br />

p =<br />

[ 1/2<br />

1<br />

]<br />

1<br />

0.5<br />

0<br />

0.5<br />

1<br />

1<br />

0.75<br />

0.5<br />

0 0.25<br />

p 1 A 11 ≥ ±p 2 A 12<br />

p 2 A 22 ≥ ±p 1 A 12<br />

0<br />

-1<br />

-0.5<br />

0.5<br />

0<br />

[ 1<br />

p =<br />

1<br />

]<br />

1<br />

0.5<br />

1<br />

1<br />

0.75<br />

0.5<br />

0 0.25<br />

0<br />

-1<br />

-0.5<br />

A 11<br />

√<br />

2A12<br />

A 22<br />

0.5<br />

0<br />

[ 2<br />

p =<br />

1<br />

]<br />

1<br />

0.5<br />

1<br />

1<br />

0.75<br />

0.5<br />

0 0.25<br />

svec ∂S 2 +<br />

Figure 42: Proper polyhedral cone K , created by intersection of halfspaces,<br />

inscribes PSD cone in isometrically isomorphic R 3 as predicted by Geršgorin<br />

discs theorem for A=[A ij ]∈ S 2 . Hyperplanes supporting K intersect along<br />

boundary of PSD cone. Four extreme directions of K coincide with extreme<br />

directions of PSD cone.

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