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arXiv 0807.1342math-phmath.MP
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指数衰减Wannier函数的紧框架

Tight frames of exponentially decaying Wannier functions

Peter Kuchment

摘要:

设L是具有周期磁势和电势的Schroedinger算子、周期介质中的Maxwell算子或R^n中任意系数关于格G为周期的自伴椭圆线性偏微分算子.也设S是其光谱的有限部分,与光谱的其余部分通过间隙分开。我们考虑了一个老问题的存在性有限的指数衰减的Wannier函数,使他们的G-移位跨越整个谱子空间对应于S在一些“好”的方式。众所周知,拓扑障碍可能存在于寻找指数衰减的Wannier函数,形成谱子空间的正交基。这种障碍具有一定的有限维(维数等于S中谱带的数目)解析向量丛的非平凡性的形式。我们表明,它总是可以找到一个有限数量的指数衰减复合Wannier函数,使他们的G-移位形成一个1紧框架的谱子空间。这似乎是最好的,当拓扑障碍存在时可以做的。函数的个数是包含“障碍丛”等价副本的平凡丛的最小维数。特别是,它与谱带的数量相一致,当且仅当丛是平凡的,在这种情况下,一个标准正交基的指数衰减复合瓦尼尔函数是已知存在的。提供了这一数字的估计数。

英文摘要:

Let L be a Schroedinger operator with periodic magnetic and electric potentials, a Maxwell operator in a periodic medium, or an arbitrary self-adjoint elliptic linear partial differential operator in R^n with coefficients periodic with respect to a lattice G. Let also S be a finite part of its spectrum separated by gaps from the rest of the spectrum. We consider the old question of existence of a finite set of exponentially decaying Wannier functions such that their G-shifts span the whole spectral subspace corresponding to S in some "nice" manner. It is known that a topological obstruction might exist to finding exponentially decaying Wannier functions that form an orthonormal basis of the spectral subspace. This obstruction has the form of non-triviality of certain finite dimensional (with the dimension equal to the number of spectral bands in S) analytic vector bundle. We show that it is always possible to find a finite number of exponentially decaying composite Wannier functions such that their G-shifts form a 1-tight frame in the spectral subspace. This appears to be the best, one can do when the topological obstruction is present. The number of the functions is the smallest dimension of a trivial bundle containing an equivalent copy of the "obstacle bundle". In particular, it coincides with the number of spectral bands if and only if the bundle is trivial, in which case an orthonormal basis of exponentially decaying composite Wannier functions is known to exist. An estimate on this number is provided.

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