We derive a framework to apply topological quantum chemistry in systems
subject to magnetic flux. We start by deriving the action of spatial symmetry
operators in a uniform magnetic field, which extends Zak's magnetic translation
groups to all crystal symmetry groups. Ultimately, the magnetic symmetries form
a projective representation of the crystal symmetry group. As a consequence,
band representations acquire an extra gauge invariant phase compared to the
non-magnetic theory. Thus, the theory of symmetry indicators is distinct from
the non-magnetic case. We give examples of new symmetry indicators that appear
at π flux. Finally, we apply our results to an obstructed atomic insulator
with corner states in a magnetic field. The symmetry indicators reveal a
topological-to-trivial phase transition at finite flux, which is confirmed by a
Hofstadter butterfly calculation. The bulk phase transition provides a new
probe of higher order topology in certain obstructed atomic insulators.Comment: 24 pages, 7 figure