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The gauge theory-formulation of string-motivated lineal gravity proposed by Cangemi and Jackiw is obtained by dimensional reduction from $(2+1)$ dimensional gravity with a Chern-Simons Lagrangian., Comment: 4 pages, no figures. Unpublished Note from 1993

Brinkmann's plane-fronted gravitational waves with parallel rays --~shortly pp-waves~-- are shown to provide, under suitable conditions, exact string vacua at all orders of the sigma-model perturbation expansion., Comment: 7 pages, no figures

The Bogoliubov particle considered in [cond-mat/0507125] admits, contrarily to the claim of the authors, an interesting Hamiltonian structure., Comment: 3 pages. Oe reference corrected

The main result of Xiao et al. [ Phys. Rev. Lett. 95, 137204 (2005)] is shown to follow from Hamiltonian mechanics., Comment: Comment. 2 pages, no figures. Reference date corrected Affiliation of the first author expanded

Recent results on the semiclassical dynamics of an electron in a solid are explained using techniques developed for ``exotic'' Galilean dynamics. The system is indeed Hamiltonian and Liouville's theorem holds for the symplectic volume form. Suitably defined quantities satisfy hydrodynamic equations., Comment: Affiliation of the first Author expanded. One more reference added. To appear in Mod. Phys. Lett. B. 5 pages

Using the non-symmetric-connection approach proposed by Osborn, we demonstrate that, for a bosonic string in a specially chosen plane-fronted gravitational wave and an axion background, the conformal anomaly vanishes at the two-loop level. Under some conditions, the anomaly vanishes at all orders., Comment: Previously not available in hep-th. Published as Physics Letters B 313, 10 (1993). Plain TeX 6 pages. No figures

The equivalence of several $SL(3)$ sigma models and their special Abelian duals is investigated in the two loop order of perturbation theory. The investigation is based on extracting and comparing various $\beta$ functions of the original and dual models. The role of the discrete global symmetries is emphasized., Comment: Plain TEX, 24 pages

A modification of the Abelian Duality transformations is proposed guaranteeing that a (not necessarily conformally invariant) $\sigma$-model be quantum equivalent (at least up to two loops in perturbation theory) to its dual. This requires a somewhat non standard perturbative treatment of the {\sl dual} $\sigma$-model. Explicit formulae of the modified duality transformation are presented for a special class of block diagonal purely metric $\sigma$-models., Comment: Latex 11 pages; remarks on a free model and references added

Various examples of target space duality transformations are investigated up to two loop order in perturbation theory. Our results show that when using the tree level (`naive') transformation rules the dual theories are in general {\it inequivalent} at two loops to the original ones, (both for the Abelian and the non Abelian duality)., Comment: 11 pages, Latex, uses espcrc2.sty

We continue the investigation of massive integrable models by means of the bootstrap fusion procedure, started in our previous work on O(3) nonlinear sigma model. Using the analogy with SU(2) Thirring model and the O(3) nonlinear sigma model we prove a similar relation between sine-Gordon theory and a one-parameter deformation of the O(3) sigma model, the sausage model. This allows us to write down a free field representation for the Zamolodchikov-Faddeev algebra of the sausage model and to construct an integral representation for the generating functions of form-factors in this theory. We also clear up the origin of the singularities in the bootstrap construction and the reason for the problem with the kinematical poles., Comment: 16 pages, revtex; references added, some typos corrected. Accepted for publication in Physical Review D

Some of the motivations for as well as the main points of the quantization of the Nappi Witten string in the light cone gauge are reviewed., Comment: 21 pages, Plain Tex, to appear in the E.P. Wigner memorial volume of Acta Physica Hungarica

The vanishing of the anomaly in the recent example of Nappi and Witten, constructed from the Wess-Zumino-Witten model based on a certain non-semisimple group, follows from a more general result valid for gravitational waves. The construction of the metric is explained.}

(Minor corrections and reference added), Comment: (ITP Budapest Report No. 502) (11 pages