Realisations of Quantum GL_p,q(2) and Jordanian GL_h,h'(2)

The quantum group GL_p,q(2) is known to be related to the Jordanian GL_h,h'(2) via a contraction procedure. It can also be realised using the generators of the Hopf algebra G_r,s. We contract the G_r,s quantum group to obtain its Jordanian analogue G_m,k, which provides a realisation of GL_h,h'(2) in a manner similar to the q-deformed case.Comment: 6 pages LaTex, Contribution to Proceedings of "8th International Colloquium on Quantum Groups and Integrable Systems", Prague, June 17 - 19, 199

Two-Parameter Differential Calculus on the h-Exterior Plane

We construct a two-parameter covariant differential calculus on the quantum $h$-exterior plane. We also give a deformation of the two-dimensional fermionic phase space.Comment: 7 page

PT-Symmetric Quantum Electrodynamics and Unitarity

More than 15 years ago, a new approach to quantum mechanics was suggested, in which Hermiticity of the Hamiltonian was to be replaced by invariance under a discrete symmetry, the product of parity and time-reversal symmetry, $\mathcal{PT}$. It was shown that if $\mathcal{PT}$ is unbroken, energies were, in fact, positive, and unitarity was satisifed. Since quantum mechanics is quantum field theory in 1 dimension, time, it was natural to extend this idea to higher-dimensional field theory, and in fact an apparently viable version of $\mathcal{PT}$-invariant quantum electrodynamics was proposed. However, it has proved difficult to establish that the unitarity of the scattering matrix, for example, the K\"all\'en spectral representation for the photon propagator, can be maintained in this theory. This has led to questions of whether, in fact, even quantum mechanical systems are consistent with probability conservation when Green's functions are examined, since the latter have to possess physical requirements of analyticity. The status of $\mathcal{PT}$QED will be reviewed in this report, as well as the general issue of unitarity.Comment: 13 pages, 2 figures. Revised version includes new evidence for the violation of unitarit