Subindex before the integral

Question

Someone knows how to display a subindex before the integral? The image shows an example, the subindex I and R it's what I'm looking for to display in Latex.

Answer 1

The amsmath package (which should be always loaded if you type more math than 1+1=2) provides the \sideset macro to add indices to the left of large operators like \sum, \prod, and (abusing it a little) \int

\documentclass{article}
\usepackage{amsmath}
\begin{document}
[
\sideset{I}{}\oint \frac{d Q}{T}
= \sideset{_I}{ i^f}\int \frac{d Q}{T}

\sideset{_R}{_i^f}\int\frac{d Q}{T}

]
\end{document}

Adding

\usepackage{newtxtext}
\usepackage[upint]{newtxmath}

uses a Times-like font closer to the picture you posted

Answer 2

You can use also leftidx package....and for the đ here there is a possible link: d with a little line through the top of it

\documentclass[a4paper,12pt]{article}
\usepackage{newtxtext,newtxmath}
\usepackage{leftidx}
\begin{document}
[\leftidx{_I}{\oint} \frac{dQ}{T}
= \leftidx{_I}{\int_i^f} \frac{dQ}{T}

\leftidx{_R}{\int_i^f}\frac{dQ}{T}]

\end{document}