%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-5-16 on page II-189 in "abraces --- Customizable over and under braces".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3examargin}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{348.0pt}

  % Shorten the vertical size of the example ...
  \AtBeginDocument{%
    \setlength\abovedisplayskip{.3\abovedisplayskip}%
    \setlength\belowdisplayskip{.3\belowdisplayskip}%
  }

%StartShownPreambleCommands
\usepackage{amsmath}  \usepackage[overload]{abraces}
\newbracespec{M}{1U1}   \newbracespec{B}{LM}   \newbracespec{e}{1R}
                        \newbracespec{b}{L1}   \newbracespec{E}{MR}
%StopShownPreambleCommands

\begin{document}
\begin{multline*}
  f(x)=a_0+a_1x+a_2x^2+
  \overbrace[B]{a_3x^3 + a_4x^4 + \dots + a_{i-1}x^{i-1} + \enspace}
               ^{\text{some explanation}}                                  \\
  \overbrace[e]{\enspace a_ix^i + a_{i+1}x^{i+1}} + \dots + a_{n-1}x^{n-1}
\end{multline*}
\end{document}