Paper detail

A separation lemma on sub-lattices

We prove that Bourgain's separation lemma, Lemma~20.14 [B2] holds at fixed frequencies and their neighborhoods, on sub-lattices, sub-modules of the dual lattice associated with a quasi-periodic Fourier series in two dimensions. And by extension holds on the affine spaces. Previously Bourgain's lemma was not deterministic, and is valid only for a set of frequencies of positive measure. The new separation lemma generalizes classical lattice partition-type results to the hyperbolic Lorentzian setting, with signature $(1, -1, -1)$, and could be of independent interest. Combined with the method in [W2], this should lead to the existence of quasi-periodic solutions to the nonlinear Klein-Gordon equation with the usual polynomial nonlinear term $u^{p+1}$.