\documentclass[11pt,fleqn]{article}
\usepackage[ngerman]{babel}
\usepackage{amsmath}
\usepackage{bbm} %Indikatorfunktion 1
\begin{document}
$\mathbbm{1}_{[0, t_j]}$
\end{document}
underCover

\documentclass[11pt,fleqn]{article}
\usepackage[ngerman]{babel}
\usepackage{amsmath}
\usepackage{bbm} %Indikatorfunktion 1
\begin{document}
$\mathbbm{1}_{[0, t_j]}$
\end{document}
\documentclass[11pt,fleqn]{article}
\usepackage[ngerman]{babel}
\usepackage{amsmath}
\usepackage{bbm} %Indikatorfunktion 1
\begin{document}
$\mathbbm{1}_{[0\,,\, t_j]}$
\end{document}