Is there a standard notation for a message concatenated with its signature? I'm wondering what's the standard (or canonical) notation for "a message along with its signature", i.e.

$$
\mathit{Message}~\Vert~\sigma_k(\mathit{Message})
\text.
$$

Here the duplicated $\mathit{Message}$ is tedious. Is there a way to put it more elegantly?
In formal papers, the standard way would be to either cleverly shorten it or to omit arguments.

For example you could write something like $M:=...$ (but please replace the dots with the actual message composition) at the end of your algorithm:
$$\operatorname{Output} (M||\sigma_K(M))$$

The alternative to this (a shortened version of your approach) would be to say that $\sigma$ is the signature in the text or define $\sigma$ somewhere and then use it as a simple variable. (outputting $(M||\sigma)$).



If you want to write a formal specification there are also ways to express a signed construct such as TLS' digitally-signed.