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    • Our study has several limitations

    2019-11-21

    Our study has several limitations. As this study was performed at a single institution, it had a limited sample size and was not powered to detect specific differences in assay performance by organ group or waitlist status. In the two CMV seropositive subjects with potential passive antibodies, the absence of detectable T PPM-18 may indicate absence of infection with CMV. Nevertheless, we were unable to confirm false positive passive CMV serology as we did not have repeat CMV serology more than 2 months after transfusion to determine if there was sero-reversion. Our gold standard of CMV serology to establish CMV infection status has limitations. Apart from variabilities among different serology assays as a reason for discordant results with T-cell analyses, previous studies have shown discordance between CMV serology and CMV-specific T-cell responses in SOT candidates even in situations where passive antibody is not an issue [17,[20], [21], [22]].
    Conclusions
    Funding This work was supported by an Alberta Innovates Health Solutions (AIHS) Collaborative Research and Innovation Opportunity (CRIO) Project Award [grant number 201201224] and a Canadian Blood Services Research and Development Award [grant number 00539]. The funding sources had no involvement in study design, writing the manuscript or in the decision to submit the article for publication.
    Competing interests
    Ethical approval
    Acknowledgements
    Introduction The motivation for this paper came about as one of the authors was writing [14]; there had been a substantial amount of recent activity on the connection between various versions of ballistic motion and purely absolutely continuous (a.c.) spectrum for 1-dimensional operators (Schrödinger, Jacobi, and CMV) [2], [11], [14], [20], [33], [34]. Since the methods of [14] produce ballistic motion for limit-periodic CMV matrices satisfying a Pastur–Tkachenko-like condition, we wanted to verify that such CMV matrices indeed had purely a.c. spectrum. Along the way, we realized that the proof of a.c. spectrum could be accomplished in a rather elegant manner by making use of spectral approximation results analogous to those known for Schrödinger operators, but which were as-yet unknown for CMV matrices. The main aim of this paper is to work out the appropriate CMV analogues of those approximation results and use them to deduce purely a.c. spectrum in the CMV analogue of the Pastur–Tkachenko class. In this application, we know that the spectrum is homogeneous in the Carleson sense, which is how we are able to deduce pure a.c. spectrum. In general, one needs to know at least that the spectrum has positive Lebesgue measure in order to deduce nontrivial conclusions from the approximation results, so, as a supplement to our work, we also establish a criterion to guarantee positive-measure spectrum for limit-periodic CMV matrices. To the best of our knowledge, this condition that ensures positive-measure spectrum is new, even for Schrödinger or Jacobi operators. An extended CMV matrix is a pentadiagonal unitary operator on with a repeating block structure of the form where and for all . Such operators arise naturally in several settings, including quantum walks in one dimension [5] and the one-dimensional ferromagnetic Ising model [8], [12]. Moreover, half-line CMV matrices, obtained by setting and restricting to arise naturally in the study of orthogonal polynomials on the unit circle (OPUC) [29], [30] and moreover are universal within the class of unitary operators having a cyclic vector (in the sense that any unitary operator with a cyclic vector is unitarily equivalent to a half-line CMV matrix). We will be particularly interested in the case in which the coefficients of are generated by an underlying dynamical system. Concretely, given a Borel probability space , a measurable μ-ergodic map (with measurable inverse), and a measurable map , one can consider for with coefficients given by One can then study as a family and, by so doing, leverage tools and techniques from dynamical systems to prove statements about for μ-almost every ω (or even every ω in some situations). This scheme subsumes many particular cases under one umbrella, such as those for which is almost-periodic (in the sense of Bohr or Bochner). In that case, Ω is a compact monothetic group and T is a translation by a topological generator of Ω. Some common examples of almost-periodic operators include: