Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Generalized Riesz systems and orthonormal sequences in Krein spaces   We analyze special classes of bi-orthogonal sets of vectors in Hilbert and in
Krein spaces, and their relations with generalized Riesz systems. In this way,
the notion of the first/second type sequences is introduced and studied. We
also discuss their relevance in some concrete quantum mechanical system driven
by manifestly non self-adjoint Hamiltonians.

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The categories of this paper are: astro-ph
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The categories of this paper are: dg-ga
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The categories of this paper are: funct-an
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The categories of this paper are: q-bio
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The categories of this paper are: mtrl-th
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The categories of this paper are: cond-mat
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The categories of this paper are: atom-ph
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The categories of this paper are: ao-sci
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The categories of this paper are: cmp-lg
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The categories of this paper are: bayes-an
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The categories of this paper are: eess
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The categories of this paper are: quant-ph
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The categories of this paper are: chao-dyn
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The categories of this paper are: nucl-th
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The categories of this paper are: q-fin
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The categories of this paper are: gr-qc