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Minimal Realizations and Determinantal Representations in the Indefinite Setting
Journal article   Peer reviewed

Minimal Realizations and Determinantal Representations in the Indefinite Setting

Joshua D. Jackson and Hugo J. Woerdeman
Integral equations and operator theory, v 94(2)
2022

Abstract

Analysis Article Mathematics Mathematics and Statistics
For a signature matrix J , we show that a rational matrix function M ( z ) that is strictly J -contractive on the unit circle T , has a strict J ~ ⊕ J -contractive realization A B C D for an appropriate signature matrix J ~ ; that is, M ( z ) = D + z C ( I - z A ) - 1 B . As an application, we use this result to show that a two variable polynomial p ( z 1 , z 2 ) of degree ( n 1 , n 2 ) , n 2 = 1 , without roots on { ( 0 , 0 ) } ∪ ( T × { 0 } ) ∪ T 2 allows a determinantal representation 1 p ( z 1 , z 2 ) = p ( 0 , 0 ) det ( I n 1 + 1 - K Z ) , Z = z 1 I n 1 ⊕ z 2 I n 2 , where K is a strict J ~ ⊕ J -contraction. This provides first evidence of a new conjecture that a two variable polynomial p ( z 1 , z 2 ) of degree ( n 1 , n 2 ) has a determinantal representation ( 1 ) with K a strict J ~ ⊕ J -contraction if and only if p ( z 1 , z 2 ) has no roots in { ( 0 , 0 ) } ∪ T 2 .

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