Double recursion

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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(nx) double recursive with respect to given functions, if

  • G(0, x) is a given function of x.
  • G(n + 1, 0) is obtained by substitution from the function G(n, ·) and given functions.
  • G(n + 1, x + 1) is obtained by substitution from G(n + 1, x), the function G(n, ·) and given functions.[1]

Robinson goes on to provide a specific double recursive function (originally defined by Rózsa Péter)

  • G(0, x) = x + 1
  • G(n + 1, 0) = G(n, 1)
  • G(n + 1, x + 1) = G(nG(n + 1, x))

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.

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