Automatic Control Principles
Wikipedia
Mason’s gain formula (MGF) is a method for finding the transfer function $H(s)$ of a linear signal-flow graph (SFG).
$$
P = \frac{\sum_{k=1}^n G_k \Delta_k}{\Delta} \
\Delta = 1 - \sum L_i + \sum L_iL_j - \sum L_i L_j L_k + … + (-1)^m\sum…+…
$$