Before revision:  For the sake of clarity, the first focus is on the two-dimensional Helmholtz equation with spatially varying wavenumber, and follows the extension to a range of operators allowing for anisotropy in the first and second order terms  . Numerical simulations illustrate the theoretical study of the new quasi-Trefftz functions.
After revision:  For the sake of clarity, the first focus is on the two-dimensional Helmholtz equation with spatially varying wavenumber, and follows the extension to a range of operators allowing for anisotropy in the first and second order terms follows . Numerical simulations illustrate the theoretical study of the new quasi-Trefftz functions.