How to create an array with both vertical and horizontal braces around the elements

Question

I want to create the matrix shown bellow in Latex.

I have the horizontal curly brackets figured out, I can use undermat{}{} for that. I do not however know how to insert the vertical curly brackets around row 1 and rows 2-4, or how to insert the centered caption bellow the array.

Can someone please help me do this?

Here is my code so far with the horizontal curly brackets.

\documentclass{article}

\newcommand\undermat[2]{%
  \makebox[0pt][l]{$\smash{\underbrace{\phantom{%
    \begin{matrix}#2\end{matrix}}}_{\text{$#1$}}}$}#2}

\begin{document}

$$
\left [
\begin{array}{cccccccc}
1.2 & 1.4 & 3.2 & 4.1 & 6.3 & 7.1 & 9.4 & 2.5 \\
1.2 & 1.4 & 3.2 & 4.1 & 6.3 & 7.1 & 9.4 & 2.5 \\
1.2 & 1.4 & 3.2 & 4.1 & 6.3 & 7.1 & 9.4 & 2.5 \\
\undermat{T1}{1.2 & 1.4 & 3.2} & \undermat{T2}{1.2 & 1.4 & 3.2} & \undermat{T3}{9.4 & 2.5} \\
\end{array}
\right ]
$$

\end{document}

Answer 1

Here's one way, using regular extensible braces:

\documentclass{article}

\usepackage{amsmath,lipsum}

\newcommand\undermat[2]{% http://tex.stackexchange.com/a/102468/5764
  \makebox[0pt][l]{$\smash{\underbrace{\phantom{%
    \begin{matrix}#2\end{matrix}}}_{\text{$#1$}}}$}#2}

\begin{document}

\lipsum*[1]
\[
  \begin{array}{@{} c @{}}
    \begin{array}{@{} r @{}}
      \text{Bias nodes}~\{\hspace{\nulldelimiterspace} \\
      \text{Nodes}~\left\{\begin{array}{@{}c@{}}\null\\\null\\\null\end{array}\right.
    \end{array}
    \left [
      \begin{array}{ *{8}{c} }
        1.2 & 1.4 & 3.2 & 4.1 & 6.3 & 7.1 & 9.4 & 2.5 \\
        1.2 & 1.4 & 3.2 & 4.1 & 6.3 & 7.1 & 9.4 & 2.5 \\
        1.2 & 1.4 & 3.2 & 4.1 & 6.3 & 7.1 & 9.4 & 2.5 \\
        \undermat{T1}{1.2 & 1.4 & 3.2} & \undermat{T2}{1.2 & 1.4 & 3.2} & \undermat{T3}{9.4 & 2.5} \\
      \end{array}
    \right ] \\
    \mathstrut
  \end{array}
\]
\lipsum[2]

\end{document}

Note the use of @{} to remove inter-column spacing. You may want to move the left construction closer to the matrix on the right using @{\hspace{-1ex}} (say). Your choice.

Since \undermat sets \underbraces that are \smashed, they add no vertical height to the matrices and therefore allows for the \left[...\right] construction to span only the visible matrix entries. However, this could cause the \undermats to run into content following the equation. To avoid this, we place the entire structure inside another array and add a blank line at the bottom. This allows the \smashed \undermats to have some whitespace to sink into.

Answer 2

Here is a solution with {bNiceMatrix} of nicematrix.

\documentclass{article}
\usepackage{nicematrix}

\begin{document}

\[
\begin{bNiceMatrix}[first-col]
\text{\itshape Bias Nodes}\hspace*{4mm} & 1.2 & 1.4 & 3.2 & 4.1 & 6.3 & 7.1 & 9.4 & 2.5 \\
                                        & 1.2 & 1.4 & 3.2 & 4.1 & 6.3 & 7.1 & 9.4 & 2.5 \\
\text{\itshape Nodes}\hspace*{4mm}      & 1.2 & 1.4 & 3.2 & 4.1 & 6.3 & 7.1 & 9.4 & 2.5 \\
                                        & 1.2 & 1.4 & 3.2 & 1.2 & 1.4 & 3.2 & 9.4 & 2.5 \\
\CodeAfter
  \SubMatrix{\{}{1-1}{1-1}{.}[xshift=1.2mm]
  \SubMatrix{\{}{2-1}{4-1}{.}[xshift=1mm]
  \UnderBrace{4-1}{4-3}{T_1}[yshift=1mm,shorten]
  \UnderBrace{4-4}{4-5}{T_2}[yshift=1mm,shorten]
  \UnderBrace{4-6}{4-8}{T_3}[yshift=1mm,shorten]
\end{bNiceMatrix}
\]

\end{document}

You need several compilations (because nicematrix uses PGF/Tikz nodes under the hood).