Concatenation Operations and Restricted Variants of Two-Dimensional Automata.

A two-dimensional automaton operates on arrays of symbols. While a standard (four-way) two-dimensional automaton can move its input head in four directions, restricted two-dimensional automata are only permitted to move their input heads in three or two directions; these models are called three-way and two-way two-dimensional automata, respectively. In two dimensions, we may extend the notion of concatenation in multiple ways, depending on the words to be concatenated. We may row-concatenate (resp., column-concatenate) a pair of two-dimensional words when they have the same number of columns (resp., rows). In addition, the diagonal concatenation operation combines two words at their lower-right and upper-left corners, and is not dimension-dependent. In this paper, we investigate closure properties of restricted models of two-dimensional automata under various concatenation operations. We give non-closure results for two-way two-dimensional automata under row and column concatenation in both the deterministic and nondeterministic cases. We further give positive closure results for the same concatenation operations on unary nondeterministic two-way two-dimensional automata. Finally, we study closure properties of diagonal concatenation on both two- and three-way two-dimensional automata.