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===Higher-order Boltzmann machines===

The stochastic dynamics and the learning rule can accommodate more
complicated energy functions (Sejnowski, 1986). For example,
the quadratic energy function in Eq. \eqref{energyfunction} can be
replaced by an energy function whose typical term is <math>s_i s_j s_k
w_{ijk}\ .</math>  The total input to unit <math>i</math> that is used in the update rule
must then be replaced by <math>z_i = b_i + \sum_{j<k} s_j s_k w_{ijk}\ .</math>
The only change in the learning rule is that <math>s_i s_j</math> is replaced by
<math>s_i s_j s_k\ .</math>