\documentclass{article}
\usepackage[utf8]{inputenc}
\newcommand{\fc}{\mathcal{F}}
\usepackage{amssymb,amsmath}

\begin{document}
\section{Base adaptée}
\begin{align*}
	\operatorname{base adaptee}\left( \bigoplus_{k} F_k \right) &:= \bigsqcup_{k} \operatorname{base}(F_k)
\end{align*}			

\begin{align*}
	\operatorname{base adaptee}\left( \underbrace{\bigsqcup_{k} \fc_k}_{\text{libre}} \right) &:= \bigoplus_{k} \operatorname{Vect}(\fc_k)
\end{align*}
\end{document}
