  The mathematical notion of spectral singularity admits a description in terms
of purely outgoing solutions of a corresponding linear wave equation. This
leads to a nonlinear generalization of this notion for nonlinearities that are
confined in space. We examine the nonlinear spectral singularities in arbitrary
TE and TM modes of a mirrorless slab laser that involves a weak Kerr
nonlinearity. This provides a computational scheme for the determination of the
laser output intensity $I$ for these modes. In particular, we offer an
essentially mathematical derivation of the linear-dependence of $I$ on the gain
coefficient $g$ and obtain an explicit analytic expression for its slope. This
shows that if the real part $\eta$ of the refractive index of the slab does not
exceed 3, there is a lower bound on $\theta$ below which lasing in both its TE
and TM modes requires $\eta$ to be shifted by a small amount as $g$ surpasses
the threshold gain. Our results suggest that lasing in the oblique TM modes of
the slab is forbidden if the incidence (emission) angle of the TM mode exceeds
the Brewster's angle.
