Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Model Companions of T_\sigma for stable T   Let T be a complete first order theory in a countable relational language L .
We assume relation symbols have been added to make each formula equivalent to a
predicate. Adjoin a new unary function symbol sigma to obtain the language
L_sigma; T_sigma is obtained by adding axioms asserting that sigma is an
L-automorphism. We provide necessary and sufficient conditions for T_sigma to
have a model companion when T is stable. Namely, we introduce a new condition:
T_sigma admits obstructions, and show that T_sigma has a model companion iff
and only if T_sigma does not admit obstructions. This condition is weakening of
the finite cover property: if a stable theory T has the finite cover property
then T_sigma admits obstructions.

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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: econ
13
The categories of this paper are: quant-ph
8
The categories of this paper are: hep-ex
12
The categories of this paper are: nucl-th
9
The categories of this paper are: nlin
29
The categories of this paper are: funct-an
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The categories of this paper are: eess
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The categories of this paper are: plasm-ph
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The categories of this paper are: chem-ph
7
The categories of this paper are: hep-th
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The categories of this paper are: ao-sci
30
The categories of this paper are: q-alg
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The categories of this paper are: math-ph
0
The categories of this paper are: hep-ph