An implicit Q-theorem for Hessenberg-like matrices

The implicit Q-theorem for Hessenberg matrices is a widespread and powerful theorem. It is used in the development of for example implicit QR-algorithms to compute the eigendecomposition of Hessenberg matrices. Moreover it can also be used to prove the essential uniqueness of orthogonal similarity transformations of matrices to Hessenberg form. The theorem is also valid for symmetric tridiagonal matrices, proving thereby also in the symmetric case its power. Currently there is a growing interest to so-called semiseparable matrices. These matrices can be considered as the inverses of tridiagonal matrices. In a similar way, one can consider Hessenberg-like matrices as the inverses of Hessenberg matrices. In this paper, we formulate and prove an implicit Q-theorem for Hessenberg-like matrices. Similarly, like in the Hessenberg case the notion of unreduced Hessenberglike matrices is introduced and also a method for transforming matrices via orthogonal transformations to this form is proposed. Moreover, as the theorem is valid for Hessenberg-like matrices it is also valid for symmetric semiseparable matrices.