A new function algebra of EXPTIME functions by safe nested recursion
  Bellantoni and Cook have given a function-algebra characterization of the polynomial-time computable functions via an unbounded recursion scheme which is called safe recursion. Inspired by their work, we characterize the exponential-time computable functions with the use of a safe variant of nested recursion. 
Toshiyasu Arai and Naohi Eguchi