146,051 research outputs found
Complexity classifications for different equivalence and audit problems for Boolean circuits
We study Boolean circuits as a representation of Boolean functions and
consider different equivalence, audit, and enumeration problems. For a number
of restricted sets of gate types (bases) we obtain efficient algorithms, while
for all other gate types we show these problems are at least NP-hard.Comment: 25 pages, 1 figur
Constraint Satisfaction with Counting Quantifiers
We initiate the study of constraint satisfaction problems (CSPs) in the
presence of counting quantifiers, which may be seen as variants of CSPs in the
mould of quantified CSPs (QCSPs). We show that a single counting quantifier
strictly between exists^1:=exists and exists^n:=forall (the domain being of
size n) already affords the maximal possible complexity of QCSPs (which have
both exists and forall), being Pspace-complete for a suitably chosen template.
Next, we focus on the complexity of subsets of counting quantifiers on clique
and cycle templates. For cycles we give a full trichotomy -- all such problems
are in L, NP-complete or Pspace-complete. For cliques we come close to a
similar trichotomy, but one case remains outstanding. Afterwards, we consider
the generalisation of CSPs in which we augment the extant quantifier
exists^1:=exists with the quantifier exists^j (j not 1). Such a CSP is already
NP-hard on non-bipartite graph templates. We explore the situation of this
generalised CSP on bipartite templates, giving various conditions for both
tractability and hardness -- culminating in a classification theorem for
general graphs. Finally, we use counting quantifiers to solve the complexity of
a concrete QCSP whose complexity was previously open
Single-machine scheduling with stepwise tardiness costs and release times
We study a scheduling problem that belongs to the yard operations component of the railroad planning problems, namely the hump sequencing problem. The scheduling problem is characterized as a single-machine problem with stepwise tardiness cost objectives. This is a new scheduling criterion which is also relevant in the context of traditional machine scheduling problems. We produce complexity results that characterize some cases of the problem as pseudo-polynomially solvable. For the difficult-to-solve cases of the problem, we develop mathematical programming formulations, and propose heuristic algorithms. We test the formulations and heuristic algorithms on randomly generated single-machine scheduling problems and real-life datasets for the hump sequencing problem. Our experiments show promising results for both sets of problems
The Complexity of Surjective Homomorphism Problems -- a Survey
We survey known results about the complexity of surjective homomorphism
problems, studied in the context of related problems in the literature such as
list homomorphism, retraction and compaction. In comparison with these
problems, surjective homomorphism problems seem to be harder to classify and we
examine especially three concrete problems that have arisen from the
literature, two of which remain of open complexity
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