\left. { + \left( {1 - q} \right)p^{k - 1} \left( {1 - p} \right)\left( {\sum\limits_{\scriptstyle J \subseteq \left\{ {1,...,k} \right\} \atop 
 {\scriptstyle \left| J \right| = k - 1 \atop 
 \scriptstyle j_1 \in J^C }} {\left( {f_1 \left( {n_{j_1 } + 1} \right)\prod\limits_{j \in J} {f_1 \left( {n_j } \right)} } \right)} } \right) + p^k \prod\limits_{i = 1}^k {f_1 \left( {n_i } \right)} } \right]