Looks like \mathcal{I}. The standard LaTeX fonts do not have a special character for the capital iota because it would look like a capital latin-i (I).
– moeweApr 05 '19 at 15:50
In unicode-math, the literal answer to the question you asked is to load unicode-math and then use \mitIota or \symit{\Iota}. Classic LaTeX math fonts use the same codepoint for I and Iota, but it is possible to use the Iota from a legacy 8-bit LGR font if you really wanted to. As @moewe observed, though, that symbol looks more like \mathcal{I}.
– DavislorApr 05 '19 at 16:23
\mathcal{I}. The standard LaTeX fonts do not have a special character for the capital iota because it would look like a capital latin-i (I). – moewe Apr 05 '19 at 15:50unicode-math, the literal answer to the question you asked is to loadunicode-mathand then use\mitIotaor\symit{\Iota}. Classic LaTeX math fonts use the same codepoint for I and Iota, but it is possible to use the Iota from a legacy 8-bit LGR font if you really wanted to. As @moewe observed, though, that symbol looks more like\mathcal{I}. – Davislor Apr 05 '19 at 16:23