Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: High Order Perturbation Theory for Spectral Densities of Multi-Particle
  Excitations: S=1/2 Two-Leg Heisenberg Ladder   We present a high order perturbation approach to quantitatively calculate
spectral densities in three distinct steps starting from the model Hamiltonian
and the observables of interest. The approach is based on the perturbative
continuous unitary transformation introduced previously. It is conceived to
work particularly well in models allowing a clear identification of the
elementary excitations above the ground state. These are then viewed as
quasi-particles above the vacuum. The article focuses on the technical aspects
and includes a discussion of series extrapolation schemes. The strength of the
method is demonstrated for S=1/2 two-leg Heisenberg ladders, for which results
are presented.

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The categories of this paper are: nucl-th
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The categories of this paper are: nlin
35
The categories of this paper are: mtrl-th
3
The categories of this paper are: physics
5
The categories of this paper are: gr-qc
28
The categories of this paper are: alg-geom
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The categories of this paper are: chao-dyn
1
The categories of this paper are: cs
2
The categories of this paper are: math
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The categories of this paper are: nucl-ex
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The categories of this paper are: q-fin
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The categories of this paper are: plasm-ph
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The categories of this paper are: patt-sol
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The categories of this paper are: math-ph
10
The categories of this paper are: q-bio
26
The categories of this paper are: bayes-an