# What does it mean for an integer to belong to the halting problem?

I have come across the description of a function $$F: \mathbb{N} \to \mathbb{N}$$ where the function is defined one way for $$n \in \mathcal{H}$$ and another way for $$n \notin \mathcal{H}.$$ In this notation $$\mathcal{H}$$ is meant to denote the halting problem.

However, I am not sure what it means for an integer to be a member of the halting problem.

Turing machines can be encoded as integers in various ways. With respect to any such encoding, we define $$\mathcal{H}$$ as the set of integers $$n$$ such that the Turing machine encoded by $$n$$ halts on the empty input.
The encoding cannot be completely arbitrary. For example, you can design such an artificial encoding in which the parity of the encoding is its halting status. Instead, we fix some natural string encoding (closely following one of the many equivalent definitions of Turing machines), and convert it into an integer encoding $$\alpha$$ is one of several natural ways (for example, convert the string into a self-delimiting binary string, which can be interpreted as a natural number). We only consider encodings $$\beta$$ such that an encoding of type $$\alpha$$ can be converted into an encoding of type $$\beta$$ of an equivalent machine, and vice versa, in a computable way (by an algorithm). The difficulty of the halting problem (in terms of computability) is the same for all such encodings (which we could call admissible).