Reversed indices

Question

How do I type ₁A in LaTeX as in the snippet below?

Answer 1

I have used newtxtext, newtxmath to obtain the same fonts of your image.

\documentclass[a4paper,12pt]{article}
\usepackage{newtxtext,newtxmath}
\begin{document}
$A=({}_{1}A, \ldots, {}_{j}A)$
\end{document}

There are different alternatives to obtain your example: using, for example,tensor package which provides for a little bit of a lower subscripts.

\documentclass[a4paper,12pt]{article}
\usepackage{newtxtext,newtxmath}
\usepackage{tensor}
\begin{document}
$A=(\tensor[_1]{A}{}, \ldots, \tensor[_j]{A}{})$
\end{document}