User blog comment:Wythagoras/All my stuff/@comment-1605058-20130708062955

Y function is far from being well defined. For example, is rule f(a)=a+a+a allowed? I assume not, but why? It uses only addition. There is also a mistake in definition of f(a,b) - middle b must be b-1, for definition not to be circular. And yes, if well defined, this function is uncomputable, because it diagonalizes through all recursive definitions.