f_2 \left( {n_1 ,n_2 } \right) = p_2 \left[ {q\left( {1 - p} \right)^2 + qp\left( {1 - p} \right)\left( {f_1 \left( {n_1 } \right) + f_1 \left( {n_2 } \right)} \right) + } \right.