| Abstract: | In the first half of the 1990s, Clote and Takeuti characterized several function complexity classes by means of the concatenation recursion on notation operators. In this paper, we borrow from computability theory well‐known techniques based on pairing functions to show that  ,  , and  functions can be characterized by means of concatenation iteration on notation. Indeed, a function class satisfying simple constraints and defined by using concatenation recursion on notation is inductively characterized by means of concatenation iteration on notation. Furthermore,  ,  , and  unary functions are inductively characterized using addition, composition, and concatenation iteration on notation. |
