Dependence of Spearman’s rank correlation coefficient on the number ratio of N_R/N_G. The symbols in blue and in dark green show the results through all the galaxy pairs and through the randomly collected 1618 galaxy pairs. The horizontal red solid line marks the coefficient to be 0.65 for all the DPN galaxies, and the horizontal red dashed lines from top to bottom show the coefficients to be 0.90, 0.88, 0.63, 0.48, and 0.31 for the galaxies in G1, G2, G4, G3, and G5, respectively.