Strictly 2-local automata are based on lookup tables that are sets of 2-factors, the pairs of adjacent symbols which are permitted to occur in a word. To generalize, we extend the 2-factors to k-factors. We now have the possibility that the scanning window is actually longer than the augmented string. To accommodate that, we will permit factors of any length up to k as long as they start with ‘x' and end with ‘x' as well as k-factors which may or may not start with ‘x' or end with ‘x'.

So a strictly k-local automaton is just an alphabet and a set of stings of length k in which the ?rst symbol is either x or a symbol of the alphabet and the last is either x or a symbol of the alphabet, plus any number of strings of length no greater than k in which the ?rst and last symbol are x and x, respectively. In scanning strings that are shorter than k - 1, the automaton window will span the entire input (plus the beginning and end symbols). In that case, it will accept i? the sequence of symbols in the window is one of those short strings.

You should verify that this is a generalization of SL2 automata, that if k = 2 the de?nition of SLk automata is the same as the de?nition of SL₂ automata.