I was looking for the three dots of therefore symbol in libreoffice writer and formula.
also the reciprocal symbol which is use for since or because.
Type 2234 then immediately Alt+X for ∴
For the upside down because 2235 Alt+x, ∵
See List of mathematical symbols by subject - Wikipedia formore unicode numbers. If you will use it more than a few times it is worth adding it to your Autocorrect options. Personally, I use it only occasionally so :therefore: works for me for autocorrect
An additional comment: for use in Math (the OP mentions “formula”) it may be useful to surround both symbols in double quotes, so they do not show in italics: a "∴" b "∵" c