My Emacs configure for YaTeX-mode, see http://mtino1594.hatenablog.com/entry/2019/04/07/200000

init.el

(require 'package)
(add-to-list 'package-archives
         '("melpa" . "https://melpa.org/packages/") t)
(package-initialize)
(unless package-archive-contents (package-refresh-contents))
(unless (package-installed-p 'use-package)
  (package-install 'use-package))
(require 'use-package)

(use-package company
  :ensure t
  :hook (after-init . global-company-mode))

(use-package flycheck
  :ensure t
  :hook (after-init . global-flycheck-mode))

(use-package yatex
  :ensure t
  :preface
  (defvar my-YaTeX-section-alist
    '(("part" . 0)
      ("chapter" . 1)
      ("section" . 2)
      ("subsection" . 3)
      ("subsubsection" . 4)
      ("paragraph" . 5)
      ("subparagraph" . 6)))

  (defvar my-YaTeX-metasection-list
    '("documentclass"
      "begin{document}" "end{document}"
      "frontmatter" "mainmatter" "appendix" "backmatter"))

  (defvar my-YaTeX-outline-regexp
    (concat (regexp-quote "\\")
        (regexp-opt (append my-YaTeX-metasection-list
                (mapcar #'car my-YaTeX-section-alist))
            t)))

  (defvar my-YaTeX-outline-promotion-headings
    '("\\chapter" "\\section" "\\subsection"
      "\\subsubsection" "\\paragraph" "\\subparagraph"))

  (defun my-YaTeX-outline-level ()
    (if (looking-at my-YaTeX-outline-regexp)
    (1+ (or (cdr (assoc (match-string 1) my-YaTeX-section-alist)) -1))
      1000))

  (defun my-YaTeX-with-outline ()
    (outline-minor-mode 1)
    (setq-local outline-regexp my-YaTeX-outline-regexp)
    (setq-local outline-level #'my-YaTeX-outline-level)
    (setq-local outline-promotion-headings my-YaTeX-outline-promotion-headings))
  :init
  (setq YaTeX-inhibit-prefix-letter t)
  :mode (("\\.tex\\'" . yatex-mode)
         ("\\.sty\\'" . yatex-mode)
         ("\\.ltx\\'" . yatex-mode))
  :hook (yatex-mode . my-YaTeX-with-outline)
  :defer t
  :config
  (setq YaTeX-kanji-code 4) ; 1 => SJIS, 2 => JIS, 3 => EUC, 4 => UTF8
  (setq YaTeX-use-AMS-LaTeX t)
  (setq tex-command "latexmk"))

(use-package company-math
  :ensure t
  :demand t
  :after (company yatex)
  :config
  (push 'company-math-symbols-latex company-backends)
  (push 'company-latex-commands company-backends))

(use-package flycheck-yatex
  :ensure nil
  ;; Asume that you download flycheck-yatex in
  ;; `~/.emacs.d/site-lisp/'.
  :load-path "site-lisp/flycheck-yatex"
  :demand t
  :after (flycheck yatex))

(use-package reftex
  :ensure nil
  :hook (yatex-mode . reftex-mode)
  :bind (:map reftex-mode-map
          ("C-c )" . nil)
          ("C-c (" . reftex-reference))
  :defer t
  :custom
  (reftex-ref-style-default-list '("Cleveref") "Use cref/Cref as default"))

(use-package outline-magic
  :ensure t
  :bind (:map outline-minor-mode-map
          ("C-<tab>" . outline-cycle)
          ("M-<left>" . outline-promote)
          ("M-<right>" . outline-demote)
          ("M-<up>" . outline-move-subtree-up)
          ("M-<down>" . outline-move-subtree-down))
  :demand t
  :after (outline))