“…Note that with d = 1, the PWL function f (x x x) is reduced to be linear, thus, we regard linear functions as a special case of PWL functions throughout the Primer. Compared to other nonlinear models, PWL functions possess explicit geometric interpretation, and many practical systems can be easily transformed into PWL nonlinear functions 37 , such as PWL memristors [G] 38,39 , specialized cost functions [40][41][42][43][44] , and part mathematical programmings [45][46][47][48][49][50] . As powerful nonlinear models, PWL functions are proven universal approximators 51 : let Ω ⊂ R n be a compact domain, and p(x x x) : Ω → R be a continuous function.…”