15,605 research outputs found
Memory lower bounds for deterministic self-stabilization
In the context of self-stabilization, a \emph{silent} algorithm guarantees
that the register of every node does not change once the algorithm has
stabilized. At the end of the 90's, Dolev et al. [Acta Inf. '99] showed that,
for finding the centers of a graph, for electing a leader, or for constructing
a spanning tree, every silent algorithm must use a memory of
bits per register in -node networks. Similarly, Korman et al. [Dist. Comp.
'07] proved, using the notion of proof-labeling-scheme, that, for constructing
a minimum-weight spanning trees (MST), every silent algorithm must use a memory
of bits per register. It follows that requiring the algorithm
to be silent has a cost in terms of memory space, while, in the context of
self-stabilization, where every node constantly checks the states of its
neighbors, the silence property can be of limited practical interest. In fact,
it is known that relaxing this requirement results in algorithms with smaller
space-complexity.
In this paper, we are aiming at measuring how much gain in terms of memory
can be expected by using arbitrary self-stabilizing algorithms, not necessarily
silent. To our knowledge, the only known lower bound on the memory requirement
for general algorithms, also established at the end of the 90's, is due to
Beauquier et al.~[PODC '99] who proved that registers of constant size are not
sufficient for leader election algorithms. We improve this result by
establishing a tight lower bound of bits per
register for self-stabilizing algorithms solving -coloring or
constructing a spanning tree in networks of maximum degree~. The lower
bound bits per register also holds for leader election
Continuity, Deconfinement, and (Super) Yang-Mills Theory
We study the phase diagram of SU(2) Yang-Mills theory with one adjoint Weyl
fermion on R^3xS^1 as a function of the fermion mass m and the compactification
scale L. This theory reduces to thermal pure gauge theory as m->infinity and to
circle-compactified (non-thermal) supersymmetric gluodynamics in the limit
m->0. In the m-L plane, there is a line of center symmetry changing phase
transitions. In the limit m->infinity, this transition takes place at
L_c=1/T_c, where T_c is the critical temperature of the deconfinement
transition in pure Yang-Mills theory. We show that near m=0, the critical
compactification scale L_c can be computed using semi-classical methods and
that the transition is of second order. This suggests that the deconfining
phase transition in pure Yang-Mills theory is continuously connected to a
transition that can be studied at weak coupling. The center symmetry changing
phase transition arises from the competition of perturbative contributions and
monopole-instantons that destabilize the center, and topological molecules
(neutral bions) that stabilize the center. The contribution of molecules can be
computed using supersymmetry in the limit m=0, and via the
Bogomolnyi--Zinn-Justin (BZJ) prescription in the non-supersymmetric gauge
theory. Finally, we also give a detailed discussion of an issue that has not
received proper attention in the context of N=1 theories---the non-cancellation
of nonzero-mode determinants around supersymmetric BPS and KK
monopole-instanton backgrounds on R^3xS^1. We explain why the non-cancellation
is required for consistency with holomorphy and supersymmetry and perform an
explicit calculation of the one-loop determinant ratio.Comment: A discussion of the non-cancellation of the nonzero mode determinants
around supersymmetric monopole-instantons in N=1 SYM on R^3xS^1 is added,
including an explicit calculation. The non-cancellation is, in fact, required
by supersymmetry and holomorphy in order for the affine-Toda superpotential
to be reproduced. References have also been adde
Leader Election in Anonymous Rings: Franklin Goes Probabilistic
We present a probabilistic leader election algorithm for anonymous, bidirectional, asynchronous rings. It is based on an algorithm from Franklin, augmented with random identity selection, hop counters to detect identity clashes, and round numbers modulo 2. As a result, the algorithm is finite-state, so that various model checking techniques can be employed to verify its correctness, that is, eventually a unique leader is elected with probability one. We also sketch a formal correctness proof of the algorithm for rings with arbitrary size
An Inflaton Mass Problem in String Inflation from Threshold Corrections to Volume Stabilization
Inflationary models whose vacuum energy arises from a D-term are believed not
to suffer from the supergravity eta problem of F-term inflation. That is,
D-term models have the desirable property that the inflaton mass can naturally
remain much smaller than the Hubble scale. We observe that this advantage is
lost in models based on string compactifications whose volume is stabilized by
a nonperturbative superpotential: the F-term energy associated with volume
stabilization causes the eta problem to reappear. Moreover, any shift
symmetries introduced to protect the inflaton mass will typically be lifted by
threshold corrections to the volume-stabilizing superpotential. Using threshold
corrections computed by Berg, Haack, and Kors, we illustrate this point in the
example of the D3-D7 inflationary model, and conclude that inflation is
possible, but only for fine-tuned values of the stabilized moduli. More
generally, we conclude that inflationary models in stable string
compactifications, even D-term models with shift symmetries, will require a
certain amount of fine-tuning to avoid this new contribution to the eta
problem.Comment: 25 page
Enhanced Symmetries and the Ground State of String Theory
The ground state of string theory may lie at a point of ``maximally enhanced
symmetry", at which all of the moduli transform under continuous or discrete
symmetries. This hypothesis, along with the hypotheses that the theory at high
energies has N=1 supersymmetry and that the gauge couplings are weak and
unified, has definite consequences for low energy physics. We describe these,
and offer some suggestions as to how these assumptions might be compatible.Comment: harvmac, 18 page
Update of D3/D7-Brane Inflation on K3 x T^2/Z_2
We update the D3/D7-brane inflation model on K3 x T^2/Z_2 with branes and
fluxes. For this purpose, we study the low energy theory including g_s
corrections to the gaugino condensate superpotential that stabilizes the K3
volume modulus. The gauge kinetic function is verified to become holomorphic
when the original N=2 supersymmetry is spontaneously broken to N=1 by bulk
fluxes. From the underlying classical N=2 supergravity, the theory inherits a
shift symmetry which provides the inflaton with a naturally flat potential. We
analyze the fate of this shift symmetry after the inclusion of quantum
corrections. The field range of the inflaton is found to depend significantly
on the complex structure of the torus but is independent of its volume. This
allows for a large kinematical field range for the inflaton. Furthermore, we
show that the D3/D7 model may lead to a realization of the recent CMB fit by
Hindmarsh et al. with an 11% contribution from cosmic strings and a spectral
index close to n_s=1. On the other hand, by a slight change of the parameters
of the model one can strongly suppress the cosmic string contribution and
reduce the spectral index n_s to fit the WMAP5 data in the absence of cosmic
strings. We also demonstrate that the inclusion of quantum corrections allows
for a regime of eternal D3/D7 inflation.Comment: LaTeX2e, 55 pages + appendices, 8 figures; v3: added appendix F and a
note at the end of the conclusions in order to clarify the relation of our
results to the recent work by Burgess et al. (arXiv:0811.1503
Supersymmetry Breaking and Dilaton Stabilization in String Gas Cosmology
In this Note we study supersymmetry breaking via gaugino condensation in
string gas cosmology. We show that the same gaugino condensate which is
introduced to stabilize the dilaton breaks supersymmetry. We study the
constraints on the scale of supersymmetry breaking which this mechanism leads
to.Comment: 11 page
Exploration of Finite 2D Square Grid by a Metamorphic Robotic System
We consider exploration of finite 2D square grid by a metamorphic robotic
system consisting of anonymous oblivious modules. The number of possible shapes
of a metamorphic robotic system grows as the number of modules increases. The
shape of the system serves as its memory and shows its functionality. We
consider the effect of global compass on the minimum number of modules
necessary to explore a finite 2D square grid. We show that if the modules agree
on the directions (north, south, east, and west), three modules are necessary
and sufficient for exploration from an arbitrary initial configuration,
otherwise five modules are necessary and sufficient for restricted initial
configurations
General considerations of the cosmological constant and the stabilization of moduli in the brane-world picture
We argue that the brane-world picture with matter-fields confined to 4-d
domain walls and with gravitational interactions across the bulk disallows
adding an arbitrary constant to the low-energy, 4-d effective theory -- which
finesses the usual cosmological constant problem. The analysis also points to
difficulties in stabilizing moduli fields; as an alternative, we suggest
scenarios in which the moduli motion is heavily damped by various cosmological
mechanisms and varying ultra-slowly with time.Comment: 5 pages, no figure
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