  Bipartite correlations in multi-qubit systems cannot be shared freely. The
presence of entanglement or classical correlation on certain pairs of qubits
may imply correlations on other pairs. We present a method of characterization
of bi-partite correlations in multi-qubit systems using a concept of entangled
graphs that has been introduced in our earlier work [M.Plesch and V.Buzek,
Phys. Rev. A 67, 012322 (2003)]. In entangled graphs each qubit is represented
by a vertex while the entanglement and classical correlations are represented
by two types of edges. We prove by construction that any entangled graph with
classical correlations can be represented by a mixed state of N qubits.
However, not all entangled graphs with classical correlations can be
represented by a pure state.
