Blackboard Bold I
Answer : If you're free to use LuaLaTeX and the unicode-math package and its \setmathfont macro, there are quite a few math fonts to choose from that provide a "double-struck" lowercase-i character. Hopefully, one of the following eight choices will appeal to you. :-) \documentclass{article} \usepackage{unicode-math} \setmainfont{Stix Two Text} % choose the text font... % Setting up eight [8!] math fonts \setmathfont{Latin Modern Math}[version=lm] \setmathfont{Cambria Math}[version=cambria] \setmathfont{Asana Math}[version=asana] \setmathfont{Stix Two Math}[version=stix2] \setmathfont{XITS Math}[version=xits] \setmathfont{TeX Gyre Termes Math}[version=termes] \setmathfont{TeX Gyre Pagella Math}[version=pagella] \setmathfont{TeX Gyre DejaVu Math}[version=dejavu] \newcommand\blurb{$\mathbb{123}\quad\mathbb{hij}\quad e^{\phi_i\mathbb{i}}$} \begin{document} \begin{tabular}{ll} Latin Modern & \mathversion{lm} \blurb\\ Cambria & \mathversio...