The irreducible weak modules for the fixed point subalgebra of the
vertex algebra associated to a non-degenerate even lattice by an automorphism
of order 2 (Part 2)
Let VL​ be the vertex algebra associated to a non-degenerate even lattice
L, θ the automorphism of VL​ induced from the −1 symmetry of
L, and VL+​ the fixed point subalgebra of VL​ under the action of
θ. In this series of papers, we classify the irreducible weak
VL+​-modules and show that any irreducible weak VL+​-module is
isomorphic to a weak submodule of some irreducible weak VL​-module or to a
submodule of some irreducible θ-twisted VL​-module. Let M(1)+
be the fixed point subalgebra of the Heisenberg vertex operator algebra M(1)
under the action of θ. In this paper (Part 2), we show that there
exists an irreducible M(1)+-submodule in any non-zero weak
VL+​-module and we compute extension groups for M(1)+.Comment: 42 pages. To appear in Journal of the Mathematical Society of Japan.
We divide the article arXiv:1910.07126 into 3 parts for publication. This is
the 2nd par