Nonlinear buckling and symmetry breaking of a soft elastic sheet sliding on a cylindrical substrate

We consider the axial compression of a thin sheet wrapped around a rigid cylindrical substrate. In contrast to the wrinkling-to-fold transitions exhibited in similar systems, we find that the sheet always buckles into a single symmetric fold, while periodic solutions are unstable. Upon further compression, the solution breaks symmetry and stabilizes into a recumbent fold. Using linear analysis and numerics, we theoretically predict the buckling force and energy as a function of the compressive displacement. We compare our theory to experiments employing cylindrical neoprene sheets and find remarkably good agreement.Comment: 20 pages, 5 figure

Some Remarks on {g\mathfrak g}-invariant Fedosov Star Products and Quantum Momentum Mappings

In these notes we consider the usual Fedosov star product on a symplectic manifold $(M,\omega)$ emanating from the fibrewise Weyl product $\circ$, a symplectic torsion free connection $\nabla$ on M, a formal series $\Omega \in \nu Z^2_{\rm\tiny dR}(M)[[\nu]]$ of closed two-forms on M and a certain formal series s of symmetric contravariant tensor fields on M. For a given symplectic vector field X on M we derive necessary and sufficient conditions for the triple $(\nabla,\Omega,s)$ determining the star product * on which the Lie derivative \Lie_X with respect to X is a derivation of *. Moreover, we also give additional conditions on which \Lie_X is even a quasi-inner derivation. Using these results we find necessary and sufficient criteria for a Fedosov star product to be $\mathfrak g$-invariant and to admit a quantum Hamiltonian. Finally, supposing the existence of a quantum Hamiltonian, we present a cohomological condition on $\Omega$ that is equivalent to the existence of a quantum momentum mapping. In particular, our results show that the existence of a classical momentum mapping in general does not imply the existence of a quantum momentum mapping.Comment: 15 pages, one corollary and one definition added to Section 4, typos remove

Dyson-Schwinger study of chiral density waves in QCD

The formation of inhomogeneous chiral condensates in QCD matter at nonzero density and temperature is investigated for the first time with Dyson-Schwinger equations. We consider two massless quark flavors in a so-called chiral density wave, where scalar and pseudoscalar quark condensates vary sinusoidally along one spatial dimension. We find that the inhomogeneous region covers the major part of the spinodal region of the first-order phase transition which is present when the analysis is restricted to homogeneous phases. The triple point where the inhomogeneous phase meets the homogeneous phases with broken and restored chiral symmetry, respectively, coincides, within numerical accuracy, with the critical point of the homogeneous calculation. At zero temperature, the inhomogeneous phase seems to extend to arbitrarily high chemical potentials, as long as pairing effects are not taken into account.Comment: 5 pages, 4 figures; v2: few minor modifications, matches published versio

The parameterized space complexity of model-checking bounded variable first-order logic

The parameterized model-checking problem for a class of first-order sentences (queries) asks to decide whether a given sentence from the class holds true in a given relational structure (database); the parameter is the length of the sentence. We study the parameterized space complexity of the model-checking problem for queries with a bounded number of variables. For each bound on the quantifier alternation rank the problem becomes complete for the corresponding level of what we call the tree hierarchy, a hierarchy of parameterized complexity classes defined via space bounded alternating machines between parameterized logarithmic space and fixed-parameter tractable time. We observe that a parameterized logarithmic space model-checker for existential bounded variable queries would allow to improve Savitch's classical simulation of nondeterministic logarithmic space in deterministic space $O(\log^2n)$. Further, we define a highly space efficient model-checker for queries with a bounded number of variables and bounded quantifier alternation rank. We study its optimality under the assumption that Savitch's Theorem is optimal

Dirac particles in Rindler space

We show that a uniformly accelerated observer experiences a "thermal" flux of Dirac particles in the ordinary Minkowski vacuum

Data driven problems in elasticity

We consider a new class of problems in elasticity, referred to as Data-Driven problems, defined on the space of strain-stress field pairs, or phase space. The problem consists of minimizing the distance between a given material data set and the subspace of compatible strain fields and stress fields in equilibrium. We find that the classical solutions are recovered in the case of linear elasticity. We identify conditions for convergence of Data-Driven solutions corresponding to sequences of ap- proximating material data sets. Specialization to constant material data set sequences in turn establishes an appropriate notion of relaxation. We find that relaxation within this Data-Driven framework is fundamentally different from the classical relaxation of energy functions. For instance, we show that in the Data-Driven framework the relaxation of a bistable material leads to material data sets that are not graphs.Comment: Result now covers the two well problem in full generality. Proof simplified. New Figure 9 illustrates geometry of separatio