41,720 research outputs found
Descriptive Complexity of Finite Structures: Saving the Quantifier Rank
Given a relational structure M on n elements, let D(M) be the minimum
quantifier rank of a first order formula identifying M up to isomorphism in the
class of n-element structures. The obvious upper bound is D(M)\le n. We show
that if the relations in M have arity at most k, then
D(M)<(1-\frac{1}{2k})n+k^2-k+2. The coefficient at n, which equals
1-\frac{1}{2k}, is probably not best possible but this is the first known bound
having it strictly below 1 (for fixed k).
If one is content to have the worse coefficient 1-\frac{1}{2k^2+2}, then one
can choose an identifying formula of a very special form: a prenex formula with
at most one quantifier alternation.
A few other results in this vein are presented.Comment: 39 page
The First Order Definability of Graphs: Upper Bounds for Quantifier Rank
We say that a first order formula A distinguishes a graph G from another
graph G' if A is true on G and false on G'. Provided G and G' are
non-isomorphic, let D(G,G') denote the minimal quantifier rank of a such
formula.
We prove that, if G and G' have the same order n, then D(G,G')\le(n+3)/2,
which is tight up to an additive constant of 1. The analogous questions are
considered for directed graphs (more generally, for arbitrary structures with
maximum relation arity 2) and for k-uniform hypergraphs.
Also, we study defining formulas, where we require that A distinguishes G
from any other non-isomorphic G'.Comment: 52 page
Chiral vortaic effect and neutron asymmetries at NICA
We study the possibility of testing experimentally signatures of P-odd
effects related with the vorticity of the medium. The Chiral Vortaic Effect is
generalized to the case of conserved charges different from the electric one.
In the case of baryonic charge and chemical potential such effect should
manifest itself in neutron asymmetries at the NICA accelerator complex measured
by the MPD detector. The required accuracy may be achieved in a few months of
accelerator running. We also discuss polarization of the hyperons and P-odd
correlations of particle momenta (handedness) as probes of vorticity.Comment: LaTeX, 10 pages, 3 figures (V2: new figure and comments added
Succinct Definitions in the First Order Theory of Graphs
We say that a first order sentence A defines a graph G if A is true on G but
false on any graph non-isomorphic to G. Let L(G) (resp. D(G)) denote the
minimum length (resp. quantifier rank) of a such sentence. We define the
succinctness function s(n) (resp. its variant q(n)) to be the minimum L(G)
(resp. D(G)) over all graphs on n vertices.
We prove that s(n) and q(n) may be so small that for no general recursive
function f we can have f(s(n))\ge n for all n. However, for the function
q^*(n)=\max_{i\le n}q(i), which is the least monotone nondecreasing function
bounding q(n) from above, we have q^*(n)=(1+o(1))\log^*n, where \log^*n equals
the minimum number of iterations of the binary logarithm sufficient to lower n
below 1.
We show an upper bound q(n)<\log^*n+5 even under the restriction of the class
of graphs to trees. Under this restriction, for q(n) we also have a matching
lower bound.
We show a relationship D(G)\ge(1-o(1))\log^*L(G) and prove, using the upper
bound for q(n), that this relationship is tight.
For a non-negative integer a, let D_a(G) and q_a(n) denote the analogs of
D(G) and q(n) for defining formulas in the negation normal form with at most a
quantifier alternations in any sequence of nested quantifiers. We show a
superrecursive gap between D_0(G) and D_3(G) and hence between D_0(G) and D(G).
Despite it, for q_0(n) we still have a kind of log-star upper bound:
q_0(n)\le2\log^*n+O(1) for infinitely many n.Comment: 41 pages, Version 2 (small corrections
Definitions with no quantifier alternation
Let be the minimum quantifier depth of a first order sentence
that defines a graph up to isomorphism. Let be the version of
where we do not allow quantifier alternations in . Define
to be the minimum of over all graphs of order . We prove that
for all we have , where
is equal to the minimum number of iterations of the binary logarithm
needed to bring to 1 or below. The upper bound is obtained by constructing
special graphs with modular decomposition of very small depth.Comment: 24 pages, we complement the lower bound proved in the first version
with a tight upper bound. The title of the paper has been change
Jordan-Wigner fermionization for spin-1/2 systems in two dimensions: A brief review
We review the papers on the Jordan-Wigner transformation in two dimensions to
comment on a possibility of examining the statistical mechanics properties of
two-dimensional spin-1/2 models. We discuss in some detail the two-dimensional
spin-1/2 isotropic XY and Heisenberg models.Comment: 19 pages, 10 figures include
Symmetries of the hypergeometric function mF_m-1
In this paper, we show that the generalized hypergeometric function mF_m-1
has a one parameter group of local symmetries, which is a conjugation of a flow
of a rational Calogero-Mozer system. We use the symmetry to construct fermionic
fields on a complex torus, which have linear-algebraic properties similar to
those of the local solutions of the generalized hypergeometric equation. The
fields admit a non-trivial action of the quaternions based on the above
symmetry. We use the similarity between the linear-algebraic structures to
introduce the quaternionic action on the direct sum of the space of solutions
of the generalized hypergeometric equation and its dual. As a side product, we
construct a ``good'' basis for the monodromy operators of the generalized
hypergeometric equation inspired by the study of multiple flag varieties with
finitely many orbits of the diagonal action of the general linear group by
Magyar, Weyman, and Zelevinsky. As an example of computational effectiveness of
the basis, we give a proof of the existence of the monodromy invariant
hermitian form on the space of solutions of the generalized hypergeometric
equation (in the case of real local exponents) different from the proofs of
Beukers and Heckman and of Haraoka. As another side product, we prove an
elliptic generalization of Cauchy identity.Comment: This is the final version of the paper. An Appendix on the
Calogero-Mozer system has been added to the previous version. The paper is to
appear in TAMS without the Appendi
The Secular Aberration Drift and Future Challenges for VLBI Astrometry
The centrifugial acceleration of the Solar system, resulting from the
gravitational attraction of the Galaxy centre, causes a phenomenon known as
'secular aberrration drift'. This acceleration of the Solar system barycentre
has been ignored so far in the standard procedures for high-precision
astrometry. It turns out that the current definition of the celestial reference
frame as epochless and based on the assumption that quasars have no detectable
proper motions, needs to be revised. In the future, a realization of the
celestial reference system (realized either with VLBI, or GAIA) should correct
source coordinates from this effect, possibly by providing source positions
together with their proper motions. Alternatively, the galactocentric
acceleration may be incorporated into the conventional group delay model
applied for VLBI data analysis.Comment: Proceedings of the "Journees-2011" meeting hold in Vienna University
of Technology, 19-21 September, 201
Metal pad instabilities in liquid metal batteries
A mechanical analogy is used to analyze the interaction between the magnetic
field, electric current and deformation of interfaces in liquid metal
batteries. It is found that, during charging or discharging, a sufficiently
large battery is prone to instabilities of two types. One is similar to the
metal pad instability known for aluminum reduction cells. Another type is new.
It is related to the destabilizing effect of the Lorentz force formed by the
azimuthal magnetic field induced by the base current and the current
perturbations caused by the local variations of the thickness of the
electrolyte layer
Some Aspects of Three-Quark Potentials
We analytically evaluate the expectation value of a baryonic Wilson loop in a
holographic model of an SU(3) pure gauge theory. We then discuss three aspects
of a static three-quark potential: an aspect of universality which concerns
properties independent of a geometric configuration of quarks; a heavy diquark
structure; and a relation between the three and two-quark potentials.Comment: 35 pages, many figures; v2: appendix and two figures added, minor
improvements in presentation; v3: improved presentation, some points
clarified, reference adde
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