A matrix with first row and column spanning over a sub-matrix

Question

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{makeidx}
\usepackage{graphicx}
   
\begin{document}

        \[
        P=\left[
        \begin{array}{cccc|cccccccccccc}
        1      & 1 & \dots    &1            & 1 & 1&  \dots       & 1 \\
        1      &   &                      &                       &   &             &   \\
        \vdots &   &        \textbf{\Huge{C}}               & &&&  \textbf{\Huge{0}}  &    \\
        1      &   &                      &                       &   &             &   \\
        \cline{2-8}
        1      &   &                      &                       &   &             &   \\
        \cdots &   &     \textbf{\Huge{0}}     &                       &   &&\textbf{\Huge{D}} \\
        1      &   &                      &                       &   &             &
        \end{array}
        \right]
        \]

\end{document}

I have written the following code. The problem I am facing is I want the line to appear after the first row. Please find the picture attached of the matrix I want to have. How do I remove the line from the first row? Please help

I would be happy and grateful if it be done by just doing some manipulations to my given code. I want to do it without using any packages

Answer 1

By adapting the code of the OP:

\documentclass{article}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
   
\begin{document}
\begin{center}
    \[
    P= 
    \left[
    \begin{matrix}
    1      & 1 & \dots                & 1 & 1                     & 1 & \dots             & 1  \\
    1      &   &                      &   & \multicolumn{1}{c|}{}  &                       &    \\
    \vdots &   & \textbf{\Huge{C}}    &   & \multicolumn{1}{c|}{}  &   & \textbf{\Huge{0}} &    \\
    1      &   &                      &   & \multicolumn{1}{c|}{}  &   &                        \\
    \cline{2-8}
    1      &   &                      &   & \multicolumn{1}{c|}{}  &   &                        \\
    \vdots &   & \textbf{\Huge{0}}    &   & \multicolumn{1}{c|}{}  &   &\textbf{\Huge{D}}       \\
    1      &   &                      &   & \multicolumn{1}{c|}{}  &   &
    \end{matrix} \;
    \right]
    \]
\end{center}
\end{document}

Answer 2

A proposition with nicematrix:

\documentclass{article}
\usepackage{nicematrix}
\usepackage{tikz}
   
\begin{document}

$\begin{pNiceArray}{ccccr}[columns-width=4mm,right-margin,nullify-dots]
1       & \Cdots & &                        & 1 \\
\Vdots  & \Block{2-2}<\Large>{C} & & \Block{2-2}<\Large>{0} &   \\
\\
  & \Block{2-2}<\Large>{0} & & \Block{2-2}<\Large>{D} &   \\
1 
\CodeAfter
\tikz \draw (4-|2) -- (4-|6) (2-|4) -- (6-|4) ;   
\end{pNiceArray}$

\end{document}

Answer 3

With array and some nesting:

\documentclass{article}
\usepackage{amsmath,array}

\newcommand{\bigentry}[2][\huge]{\begin{tabular}{@{}c@{}}#1$#2$\end{tabular}}

\begin{document}

\[
\newcommand{\ones}{\begin{matrix} 1 \\[-1ex] \vdots \\ 1 \end{matrix}}
\left[\begin{array}{ c @{} c | c @{} }
\noalign{\vspace{0.5ex}}
\begin{matrix} 1 \end{matrix} &
\multicolumn{1}{c@{}}{\begin{matrix} 1 & \dots & 1 \end{matrix}} &
\begin{matrix} 1 & \dots & 1 \end{matrix} \\
\ones & \bigentry{\mathbf{C}} & \bigentry{\mathbf{0}} \\
\cline{2-3}
\ones & \bigentry{\mathbf{0}} & \bigentry{\mathbf{D}} \\
\noalign{\vspace{0.5ex}}
\end{array}\right]
\]

\end{document}

We need a couple of tricks because of \vdots not being really suitable for the purpose, so some vertical space has to be added at the top and the bottom.

Avoid \textbf for math objects in formulas, it should be \mathbf.

The nested tabular for \bigentry ensures vertical centering and can accommodate a size changing command, which would otherwise be illegal in math mode.

The command \ones is just a local shorthand for reducing code clutter.

Answer 4

Like this? I use nicematrix package to redraw the matrix, the key point is first-row option.

\documentclass{article}
\usepackage{nicematrix}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}

\begin{document}
\[
p=\left[
\begin{NiceArray}{*4{C}|*3{C}}[first-row, extra-margin=2mm]
  1 & 1 & \Cdots & 1 & 1 & \Cdots & 1 \\
  1 & \Block{3-3}<\Huge>{\mathbf{C}} & & & \Block{3-3}<\Huge>{\mathbf{0}} & & \\
  \Vdots & & & & & & \\
  1 & & & & & & \\
\cline{2-7}
  1 & \Block{3-3}<\Huge>{\mathbf{0}} & & & \Block{3-3}<\Huge>{\mathbf{D}} & & \\
  \Vdots & & & & & & \\
  1 & & & & & & \\
\end{NiceArray}\right]
\]
\end{document}