Figure 3. Degree correlations in directed scale-free networks.

The Pearson correlation coefficient r ⁺⁻ is always negative, indicating that directed scale-free networks are naturally disassortative when one considers the out-in correlation. The inset shows a contour plot of the same data, for added clarity. The labels in the contour plot indicate the value of r ⁺⁻ for the corresponding contour lines.