X-matrices as a platform for studying multipartite entanglement

We derive an algebraic formula for the genuinely multipartite entanglement of a N-qubit X state. The entanglement dynamics of a N-qubit GHZ-state is a decoherence scenario. We also show that the mixedness of a state limits its entanglement, and the critical mixedness, beyond which no X-state is entangled, increases with the number of qubits.