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Consider a binary stochastic variable $D$ with possible states on and off, denoted by $D^1$ and $D^0$ . The state of $D$ is informed by $n$ independent binary stochastic variables $x_0$ through $x_n$, also with possible states on and off.

When $x_i$ is on it sends probability $f_i$ to $D$, which is the probability that $D$ is on given $x_i$:

$$P(D^1 | x_i) = f_i$$ $$P(D^0 | x_i) = 1 - f_i$$

When multiple $x$ are on simultaneously then $D$ receives multiple probabilities; if we consider these as independent their product gives their joint probability, which we define as being the final probability for state $D^1$:

$$ P(D^1) = \prod x_i f_i $$

(x is defined as having value 0 or 1 in states off and on respectively)

The models parameters are the $f_i$, hence we can train the model by following the gradient at each $f_i$:

$$ \partial_{f_i} P(D^1) = \partial_{f_i} \prod_i x_i f_i $$ $$ \partial_{f_i} P(D^1) = \left(\prod_{j \neq i} x_j f_j \right) x_i $$

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