Unicity Concepts for Sudoku
  This paper deals with a generalized Sudoku problem and investigates the unicity of a given solution. We introduce constraint sets, which is a generalization of the rows, columns and blocks of a classical Sudoku puzzle. The unicity property is characterized by three different properties. We describe unicity by permutations, by unicity cells and by rectangles. These terms are defined in this paper and are illustrated with examples. Throughout this paper we are not concerned with the existence of a solution. 
Thomas Fischer
math.CO