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The complexity of conservative valued CSPs.

Fri, 10/14/2011 - 15:01 — SomNambul
Authors: Vladimir Kolmogorov, Stanislav Zivny
Subjects: Computational Complexity
link: http://arxiv.org/abs/1110.2809
Abstract

We study the complexity of valued constraint satisfaction problems (VCSP). A
problem from VCSP is characterised by a \emph{constraint language}, a fixed set
of cost functions over a finite domain. An instance of the problem is specified
by a sum of cost functions from the language and the goal is to minimise the
sum.

We consider the case of languages containing all possible unary cost
functions. In the case of languages consisting of only $\{0,\infty\}$-valued
cost functions (i.e. relations), such languages have been called
\emph{conservative} and studied by Bulatov [LICS'03] and recently by Barto
[LICS'11]. Since we study valued languages, we call a language conservative if
it contains all finite-valued unary cost functions. The complexity of
conservative valued languages has been studied by Cohen et al. [AIJ'06] for
languages over Boolean domains, by Deineko et al. [JACM'08] for
$\{0,1\}$-valued languages (a.k.a Max-CSP), and by Takhanov [STACS'10] for
$\{0,\infty\}$-valued languages containing all finite-valued unary cost
functions (a.k.a. Min-Cost-Hom).

We prove a Schaefer-like dichotomy theorem for conservative valued languages:
if all cost functions in the language satisfy a certain condition (specified by
a complementary combination of \emph{STP and MJN multimorphisms}), then any
instance can be solved in polytime (via a new algorithm developed in this
paper), otherwise the language is NP-hard. This is the \emph{first} complete
complexity classification of \emph{general-valued constraint languages} over
non-Boolean domains.

This generalises previous results by Takhanov [STACS'10] and (a subset of
results) by Cohen et al. [AIJ'06] and Deineko et al. [JACM'08]. Moreover, our
results do not rely on any computer-assisted search as in Deineko et al.
[JACM'08], and provide a powerful tool for proving hardness of finite- and
general-valued languages.

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