Double recursion
In recursive function theory, double recursion is an extension of primitive recursion which allows the definition of non-primitive recursive functions like the Ackermann function. Raphael M. Robinson called functions of two natural number variables "G"("n", "x") double recursive with respect to "given functions", if Robinson goes on to provide a specific double recursive function (originally defined by Rózsa Péter) where the "given functions" are primitive recursive, but "G" is not primitive recursive. In fact, this is precisely the function now known as the Ackermann function.