/usr/include/deal.II/multigrid/ is in libdeal.ii-dev 8.4.2-2+b1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/deal.II/multigrid/mg_base.h
/usr/include/deal.II/multigrid/mg_block_smoother.h
/usr/include/deal.II/multigrid/mg_coarse.h
/usr/include/deal.II/multigrid/mg_constrained_dofs.h
/usr/include/deal.II/multigrid/mg_dof_accessor.h
/usr/include/deal.II/multigrid/mg_matrix.h
/usr/include/deal.II/multigrid/mg_smoother.h
/usr/include/deal.II/multigrid/mg_tools.h
/usr/include/deal.II/multigrid/mg_transfer.h
/usr/include/deal.II/multigrid/mg_transfer.templates.h
/usr/include/deal.II/multigrid/mg_transfer_block.h
/usr/include/deal.II/multigrid/mg_transfer_block.templates.h
/usr/include/deal.II/multigrid/mg_transfer_component.h
/usr/include/deal.II/multigrid/mg_transfer_component.templates.h
/usr/include/deal.II/multigrid/mg_transfer_matrix_free.h
/usr/include/deal.II/multigrid/multigrid.h
/usr/include/deal.II/multigrid/multigrid.templates.h
/usr/include/deal.II/multigrid/sparse_matrix_collection.h