54A10 2 54D35 2 Bohr compactification 2 semigroup of matrix units 2 semitopological semigroup 2 topological semigroup 2 20M15 1 20M18 1 20M18. Secondary 20M05 1 22A15 1 22A26 1 54C25 1 54D25 1 54D30 1 54D45 1 54H10 1 54H11 1 Primary 22A15 1 \(H\)-closed semigroup 1 absolutely \(H\)-closed semigroup 1 algebraically \(h\)-closed semigroup 1 bicyclic monoid 1 countably compact 1 embedding 1 feebly compact 1 free monoid 1 inverse semigroup 1 locally compact 1 polycyclic monoid 1 topological inverse semigroup 1