%&LaTeX
\documentclass{article}

\usepackage[latin1]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{textcomp}

\begin{document}

\begin{thebibliography}{1}

\bibitem{Piperno+Tronci1989} Adolfo Piperno, and Enrico Tronci. "Regular Systems of Equations in $\lambda$-calculus." In \textit{Ictcs}. Mantova - Italy, 1989. DOI: 10.1142/S0129054190000230.

\end{thebibliography}

\end{document}

