In LyX, how to add pseudo-headers to a matrix with brackets?

Question

I have this matrix in LyX:

LyX generates this latex code:

\documentclass[english]{article}
\usepackage[T1]{fontenc}
\usepackage[latin9]{inputenc}
\usepackage{amsmath}
\usepackage{babel}
\begin{document}
  \begin{align}
    \mathcal{M}(T)=
    \begin{pmatrix}
      &  & A_{1,k}\
      &  & \vdots\
      &  & A_{m,k}
    \end{pmatrix}
  \end{align}
\end{document}

I'd like to extend the matrix to this:

How can I do using LyX GUI?

Answer 1

Here's a way to do it with Lyx.

As far as I can see both from checking out Lyx 2.3.7, and internet search, there are no GUI-action prepared for what you want to do. I tried two things.

First, which is close to what you want to do by mouse:

• M=
• Alt + M (
• insert Matrix 3*3
• which creates the first row
• as you can see, it wraps an array environment into math mode ($ .. $)

Second: There was a similar question, which was answered in Latex-terms. Actions:

• insert TEX code (crtl + L)
• put math mode $$ or [ ] depending on your goal
• copy said code (or write it down here by hand) into the red frame

What's missing is specifying the package:

• Document/Settings
• LaTeX Preamble
• enter the package call from said answer

From watching the LaTeX (pdflatex) preview window I know, I copied as it should be.

Compiling (ctrl + r, and you're done) I have to do in a different way, as my setup is still quite immature:

• save
• File/Export/Latex (pdflatex)
• open the new .tex file in my current Latex environment (Texmaker)
• compile

There I am:

Lyx-file: This is how Lyx stores results from GUI-actions; past this code into a file, say yours.lyx, and open it with Lyx to see my edits:

#LyX 2.3 created this file. For more info see http://www.lyx.org/
\lyxformat 544
\begin_document
\begin_header
\save_transient_properties true
\origin unavailable
\textclass article
\begin_preamble
\usepackage[renew-dots,renew-matrix]{nicematrix}
\end_preamble
\use_default_options true
\maintain_unincluded_children false
\language english
\language_package default
\inputencoding auto
\fontencoding global
\font_roman "default" "default"
\font_sans "default" "default"
\font_typewriter "default" "default"
\font_math "auto" "auto"
\font_default_family default
\use_non_tex_fonts false
\font_sc false
\font_osf false
\font_sf_scale 100 100
\font_tt_scale 100 100
\use_microtype false
\use_dash_ligatures true
\graphics default
\default_output_format default
\output_sync 0
\bibtex_command default
\index_command default
\paperfontsize default
\spacing single
\use_hyperref false
\papersize default
\use_geometry false
\use_package amsmath 1
\use_package amssymb 1
\use_package cancel 1
\use_package esint 1
\use_package mathdots 1
\use_package mathtools 1
\use_package mhchem 1
\use_package stackrel 1
\use_package stmaryrd 1
\use_package undertilde 1
\cite_engine basic
\cite_engine_type default
\biblio_style plain
\use_bibtopic false
\use_indices false
\paperorientation portrait
\suppress_date false
\justification true
\use_refstyle 1
\use_minted 0
\index Index
\shortcut idx
\color #008000
\end_index
\secnumdepth 3
\tocdepth 3
\paragraph_separation indent
\paragraph_indentation default
\is_math_indent 0
\math_numbering_side default
\quotes_style english
\dynamic_quotes 0
\papercolumns 1
\papersides 1
\paperpagestyle default
\tracking_changes false
\output_changes false
\html_math_output 0
\html_css_as_file 0
\html_be_strict false
\end_header
\begin_body
\begin_layout Standard
M=
\begin_inset Formula $\left(\begin{array}{ccc}
 & a & b\
c & 1 & 2\
d & 3 & 4
\end{array}\right)$
\end_inset
\end_layout
\begin_layout Standard
\begin_inset ERT
status open
\begin_layout Plain Layout
$M=
\backslash
begin{pmatrix}[first-row,first-col]
\end_layout
\begin_layout Plain Layout
& a & b 
\backslash
\backslash
\end_layout
\begin_layout Plain Layout
x & 1 & 2 
\backslash
\backslash
\end_layout
\begin_layout Plain Layout
y & 3 & 4
\end_layout
\begin_layout Plain Layout
\backslash
end{pmatrix}$
\end_layout
\end_inset
\end_layout
\end_body
\end_document

Compared to Texmaker or any other Latex setup

All I then need to do is:

• creating a new file
• copy code from said answer (or type it manually)
• save
• compile

Time and again, GUIs seem to be neat, but add a lot of new complexity, often paired with little flexibility ... while knowing Latex by heart, having a nice supporting editor and some command line based compille will be fast, less error prone, keep you going.

Answer 2

An alternative to @MS-SPO solution without using an additional package, and then no need to create/edit the preamble file. The code being not standard for LyX (there is no dedicated menu entry), it must be entered manually, i.e. using the so-called ERT box.

$
\bordermatrix {
       & v_1 & \dots   & v_k & \dots & v_n \cr
w_1    &     &         & A_{1,k} \cr
\vdots &     &         & \vdots \cr
w_m    &     &         & A_{m,k} \cr
}
$

There is a slight problem in the vertical alignment of the vertical headers, but I didn't find the documentation for \bordermatrix and don't know if there are options to fix it without adding an extra space.