Degree correlations in directed scale-free networks.

<p>The Pearson correlation coefficient <i>r</i>⁺⁻ 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 <i>r</i>⁺⁻ for the corresponding contour lines.