  We discuss a strategy to construct gapped boundaries for a large class of
symmetry-protected topological phases (SPT phases) beyond group cohomology.
This is done by a generalization of the symmetry extension method previously
used for cohomological SPT phases. We find that this method allows us to
construct gapped boundaries for time-reversal-invariant bosonic SPT phases and
for fermionic Gu-Wen SPT phases for arbitrary finite internal symmetry groups.

Strongly Correlated Electrons;High Energy Physics - Theory