TeX source:

I(x_i;y_j) \triangleq \log_2\left(\frac{1}{\Pr_X\{x_i\}}\right) - \log_2\left(\frac{1}{\Pr_{X|Y}\{x_i|y_j\}}\right) = \log_2\left(\frac{\Pr_{X|Y}\{x_i|y_j\}}{\Pr_X\{x_i\}}\right).