TeX source:

|\Psi\rangle\langle\Psi|=(\sum_i|i\rangle_A\otimes|i\rangle_B)(\sum_j\langle j|_A\otimes\langle j|_B) = \color{red}{\sum_{i,j}|i\rangle_A\langle j|_A \otimes |i\rangle_B\langle j|_B}.