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arXiv 1503.05003math.CA
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CMV矩阵的达布变换

Darboux transformations for CMV matrices

M. J. Cantero, F. Marcellán, L. Moral, L. Velázquez

摘要:

我们发展了一个理论的达布变换CMV矩阵,规范表示的酉算子。在完美的类比与他们的自伴版本-雅可比矩阵的达布变换-他们相当于洛朗多项式修改的基本措施。我们解决其他问题,强调雅可比和CMV矩阵的达布变换之间的相似性,像他们的(几乎)isospectrality或关系,他们之间建立相应的正交多项式,也表明这两个变换连接的Szegö映射。然而,我们发现的CMV矩阵的达布变换的一些功能是在与雅可比的情况下形成鲜明对比。特别是,当应用到CMV矩阵,矩阵实现的逆达布变换-我们称之为“达布变换参数”-导致虚假的解决方案,其解释值得今后的研究。这样的伪解既不是酉矩阵也不是带状矩阵,所以CMV矩阵的达布变换比它们的雅可比变换更容易受到无限矩阵代数的微妙影响。无限矩阵的Cholesky分解在我们的理论中扮演了一个关键的角色。实际上,本文所介绍的达布变换是基于CMV矩阵上的一阶厄米特洛朗多项式的Cholesky分解。我们还展示了如何这些变换可以推广到更高程度的洛朗多项式,以及CMV矩阵的扩展到准定泛函-我们称之为“准CMV”矩阵。

英文摘要:

We develop a theory of Darboux transformations for CMV matrices, canonical representations of the unitary operators. In perfect analogy with their self-adjoint version -- the Darboux transformations of Jacobi matrices -- they are equivalent to Laurent polynomial modifications of the underlying measures. We address other questions which emphasize the similarities between Darboux transformations for Jacobi and CMV matrices, like their (almost) isospectrality or the relation that they establish between the corresponding orthogonal polynomials, showing also that both transformations are connected by the Szegö mapping. Nevertheless, we uncover some features of the Darboux transformations for CMV matrices which are in striking contrast with those of the Jacobi case. In particular, when applied to CMV matrices, the matrix realization of the inverse Darboux transformations -- what we call 'Darboux transformations with parameters' -- leads to spurious solutions whose interpretation deserves future research. Such spurious solutions are neither unitary nor band matrices, so Darboux transformations for CMV matrices are much more subject to the subtleties of the algebra of infinite matrices than their Jacobi counterparts. A key role in our theory is played by the Cholesky factorizations of infinite matrices. Actually, the Darboux transformations introduced in this paper are based on the Cholesky factorizations of degree one Hermitian Laurent polynomials evaluated on CMV matrices. We also show how these transformations can be generalized to higher degree Laurent polynomials, as well as to the extension of CMV matrices to quasi-definite functionals -- what we call 'quasi-CMV' matrices.

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