User blog comment:LittlePeng9/The largest number ever/@comment-35254569-20190228160407/@comment-35470197-20190228223109

There is no minus fnction symbol in Robinson arithmetic. But if you define the minus in a usual way using addition, \(N - 1\) is just \(N\) by the definition.

I note that \(N\) is not a standard natural number, but a transcendental one in a specific model of Robinson arithmetic, which does not satisfy Peano arithmetic. Therefore it is not definable in Robinson arithmetic. He considered another formal theory obtained as Robinson arithmetic and a constant term symbol interpreted by \(N\).