Mathematical Alphanumerical capital I symbol

I'm looking to type this

that I saw in a pdf document. However, when I google the mathematical alphanumerical symbols on Wikipedia, it has symbol styles for other letters, but I seems to be reserved.

Any ideas?

• Try to use "\mathcal". Mar 18 '20 at 22:31
• A good question (but not appropriate here) would be why these Unicode symbols are missing. ... Sometimes people say: I don't need to learn TeX, everything I need is already in Unicode. I seems Afro found differently. Mar 19 '20 at 15:24

Use \mathcal I_1 to get $$\mathcal I_1$$.
For example U+2110 is "Script Capital I" and looks like ℐ allowing U+1D4A4 to be ignored
while U+210E is "Planck Constant" and looks like ℎ allowing U+1D455 to be ignored