(frame decoration) with an arrow tip. As far as I could search in the document recommended by you, such a bracket does not exist in LaTeX.
Paul - did you mean something like \underset{\{\}}{\rightarrow} ?? You can get this while in math mode by typing \underset and then inserting the appropriate entries..
No, Nirmal. I mean the following:
$$\underbracket[0.7pt][1em]{\mathbb{R}\longrightarrow\mathbb{R}}$$
for example (a bit tricky ...)
\[\underset{\xrightarrow{\hphantom{R\longrightarrow R}}}{R\longrightarrow R}\]
Herbert
