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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    BIT 39 (1999), S. 757-779 
    ISSN: 1572-9125
    Keywords: Overdetermined linear systems ; rank reduction ; Hankel structure ; Toeplitz structure ; structured total least norm ; total least squares ; 1-norm ; 2-norm ; singular value decomposition
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract The structure preserving rank reduction problem arises in many important applications. The singular value decomposition (SVD), while giving the closest low rank approximation to a given matrix in matrix L 2 norm and Frobenius norm, may not be appropriate for these applications since it does not preserve the given structure. We present a new method for structure preserving low rank approximation of a matrix, which is based on Structured Total Least Norm (STLN). The STLN is an efficient method for obtaining an approximate solution to an overdetermined linear system AX ≈ B, preserving the given linear structure in the perturbation [E F] such that (A + E)X = B + F. The approximate solution can be obtained to minimize the perturbation [E F] in the L p norm, where p = 1, 2, or ∞. An algorithm is described for Hankel structure preserving low rank approximation using STLN with L p norm. Computational results are presented, which show performances of the STLN based method for L 1 and L 2 norms for reduced rank approximation for Hankel matrices.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1432-0533
    Keywords: Key words Reperfusion ; Cerebral blood flow ; Vascular smooth muscle cell ; Pericyte ; Scanning ; electron microscopy
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The present study was undertaken to ascertain the role of smooth muscles and pericytes in the microcirculation during hyperperfusion and hypoperfusion following ischemia in rats. Paired external carotids, the pterygopalatine branch of the internal carotids and the basilar artery were exposed and divided. Reversible inflatable occluders were placed around the common carotids. After 24 h, the unanesthetized rat underwent 10-min ischemia by inflating the occluders. Continuous cortical cerebral blood flow (c-CBF) was monitored by laser Doppler flowmetry. The measured c-CBF was below 20% of control (P 〈 0.001) during ischemia. A c-CBF of 227.5 ± 54.1% (P 〈 0.001) was obtained during reperfusion hyperemia. A c-CBF of 59.7 ± 8.8% (P 〈 0.001) occurred at the nadir of postischemic hypoperfusion, and this was followed by a second hyperemia. The cytoarchitecture of the vascular smooth muscles and pericytes was assessed by scanning electron microscopy. Samples were prepared using a KOH-collagenase digestion method. In control rats, arteriolar muscle cells showed smooth surfaces. Capillary pericytes were closely apposed to the endothelium. Immediately after reperfusion, transverse membrane creases were observed on the smooth muscle surfaces. During maximal hyperemia the creases disappeared. When c-CBF started to decrease the creases became visible again. Throughout the postischemic hypoperfusion the creases remained. Capillary endothelial walls became tortuous in the late phase of hypoperfusion. During the second hyperemia most arteriolar muscle cells showed smooth surfaces. Some pericytes appeared to have migrated from the vascular wall. The morphological changes of smooth muscle membranes suggest that they are related to specific perfusional disturbances during ischemia and reperfusion.
    Type of Medium: Electronic Resource
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