TeX source:

I(x_i;y_j) = \log_2\left(\frac{\Pr_{X|Y}\{x_i|y_j\}}{\Pr_X\{x_i\}}\right) = \log_2\left(\frac{\Pr_{Y|X}\{y_j|x_i\}}{\Pr_Y\{y_j\}}\right) = I(y_j;x_i),