Given a cell's state and the state of its neighbors at time t, the rule determines the cell's state at time t + 1.

There are many, but one plausible rule is based on the idea that weight and threshold modification should be determined by a fraction of (t - y).

My father, who was trained in engineering at M. I. T. in the slide-rule era, often lamented the way the pocket calculator, for all its convenience, diminished my generation's math skills.