  Closed formulae for upper bound on three tangles of three-qubit reduced
states in terms of three-qubit invariant polynomials of pure four-qubit states
are obtained. Our results offer tighter constraints on total three-way
entanglement of a given qubit with the rest of the system than those used in
ref. [PRL 113, 110501 (2014), PRL 116, 049902(E) (2016)] to verify monogamy of
four-qubit quantum entanglement.
