How can I create a calligraphic 1 (mathematical indicator) in formula mode?
What do you mean? A label for referencing the formula in text? Remember that Math is rather a utility than an “full” application by itself. I mean it provides math formulas to be inserted in Writer, Calc, Draw or Impress.
Best to do: edit your question (=modify it, don’t answer with a comment) to provide an illustrated example (for instance, draw it on a sheet of paper and attach a picture of it).
Don’t forget to mention OS name, LO version and save format.
If I look in Writer’s Special Characters dialogue at Cambria Math font which comes with Microsoft something, and search on “double-struck” there is a Mathematical Double-struck Digit One, 𝟙, U+1D7D9. Insert it into Writer, copy from there and paste into Math.
The trick is to find a font with 𝟙 included; Windows’ Segoe UI Symbol also includes it but there are licensing restrictions on all Segoe UI fonts.
If you use it frequently, you could Add it to the Special set in Symbols in Math, Edit Symbols
I see that “Google fonts” provides the Noto Sans Math font under an open license, looks like it is free for both personal and professional/commercial use, and also provides a reasonable set of international characters so you can most likely use it as a drop-in replacement math font.
The main drawback I see with this is the “sans serif”. For some characters in some contexts, the serifs are important for good looks and readability.
I cannot find any freely available “serif” font which provides both the requested character and what I see as necessary letters from my national alphabet. I have not searched extensively. If you are OK with using only the characters from the English alphabet, you may have more luck in searching.
I see the indicator function is often typeset with a solid, bold 1. Perhaps this is a useable workaround for you, something like :
{bold 1}_{T union G} = ...
To me, this looks better than when I use the sans serif font I found with
𝟙_{T union G} = ...
The latter looks OK in Windows with default setup (no font manipulation necessary), but if this is for publishing you may run into licensing issues, as mentioned by @EarnestAl .
It looks like FreeSerif contains double-struck digits at U+1D7d8 to U+1D7E1 but strokes are symmetric (contrary to your picture where left stroke is heavier than right stroke).
TeXGyreTermesMath also has serif double-struck digits but again without the dissymmetrical effect you’re looking for.
Both are free fonts.
Note: there is no need to enter the double-struck digit in Writer first, then copy and paste it into Math. You can type directly “U+1D7D9” (without the quotes) followed by Alt+X in Math. This Unicode-escape trick is valid in all LO components.
PS: This Stack Exchange question displays a sample of math fonts. The LATEX “px” font seems to offer what you’re looking for. Though, I have no idea how you can install this font for LO usage nor about licence issues.
some examples to copy any character:
math double struck_LiesMich.odt (11.8 KB)
math double struck_PROBE.odf (6.5 KB)
math double struck_PROBE.pdf (61.1 KB)
math double struck_U+nnnnnn-UNiCODE.pdf (130.7 KB)
math double struck_U+nnnnnn-UNiCODE.ods (24.9 KB)