Support the "\[..\]" and "$$..$$" Org LaTeX fragment syntax too

example-site/content-org/all-posts.org

@@ -690,22 +690,45 @@ This gets both categories =catA= and =catB=.
 :EXPORT_FILE_NAME: equation-latex-frag
 :EXPORT_DATE: 2017-07-31
 :END:
-Wrap the equations between =\(= and =\)=.
+- Inline equations are wrapped between =\(= and =\)=.
+  - =$= wrapping also works, but it is not preferred as it comes with
+    restrictions like "there should be no whitespace between the
+    equation and the =$= delimiters".
+
+    So =$ a=b $= will not work (it will look like: $ a=b $), but
+    =$a=b$= will work (it will look like: $a=b$).
+
+    On the other hand, both =\(a=b\)= (it will look like: \(a=b\)) and
+    =\( a=b \)= (it will look like: \( a=b \)) will work.
+- One-per-line equations are wrapped between =\[= and =\]= or =$$=
+  delimiters.
 
 For example, below in Org:
 #+BEGIN_SRC text
 LaTeX formatted equation: \( E = -J \sum_{i=1}^N s_i s_{i+1} \)
 #+END_SRC
 
-will look like this in Hugo rendered HTML (don't see this in Markdown,
-see what it looks after Hugo has processed it).
+will look like this in Hugo rendered HTML:
 
 LaTeX formatted equation: \( E = -J \sum_{i=1}^N s_i s_{i+1 }\)
 
-Here's another example similar to one in [[http://orgmode.org/manual/LaTeX-fragments.html][(org) LaTeX fragments]].
+(Don't see this in Markdown, see what it looks after Hugo has
+processed it.)
+
+Here's another example, taken from [[http://orgmode.org/manual/LaTeX-fragments.html][(org) LaTeX fragments]].
 
+Below in Org:
+#+BEGIN_EXAMPLE
 If $a^2=b$ and \( b=2 \), then the solution must be either
-$a=+\sqrt{2}$ or \(a=-\sqrt{2}\).
+$$ a=+\sqrt{2} $$ or \[ a=-\sqrt{2} \]
+#+END_EXAMPLE
+
+renders to:
+
+If $a^2=b$ and \( b=2 \), then the solution must be either
+$$ a=+\sqrt{2} $$ or \[ a=-\sqrt{2} \]
+
+(Note that the last two equations show up on their own lines.)
 * TODO Pre-Draft State
 :PROPERTIES:
 :EXPORT_FILE_NAME: draft-state-todo

example-site/content/posts/equation-latex-frag.md

@@ -5,20 +5,44 @@ tags = ["equations"]
 draft = false
 +++
 
-Wrap the equations between `\(` and `\)`.
+-   Inline equations are wrapped between `\(` and `\)`.
+    -   `$` wrapping also works, but it is not preferred as it comes with
+        restrictions like "there should be no whitespace between the
+        equation and the `$` delimiters".
+
+        So `$ a=b $` will not work (it will look like: $ a=b $), but
+        `$a=b$` will work (it will look like: \\(a=b\\)).
+
+        On the other hand, both `\(a=b\)` (it will look like: \\(a=b\\)) and
+        `\( a=b \)` (it will look like: \\( a=b \\)) will work.
+-   One-per-line equations are wrapped between `\[` and `\]` or `$$`
+    delimiters.
 
 For example, below in Org:
 
 ```text
 LaTeX formatted equation: \( E = -J \sum_{i=1}^N s_i s_{i+1} \)
 ```
 
-will look like this in Hugo rendered HTML (don't see this in Markdown,
-see what it looks after Hugo has processed it).
+will look like this in Hugo rendered HTML:
 
 LaTeX formatted equation: \\( E = -J \sum\_{i=1}^N s\_i s\_{i+1 }\\)
 
-Here's another example similar to one in [(org) LaTeX fragments](http://orgmode.org/manual/LaTeX-fragments.html).
+(Don't see this in Markdown, see what it looks after Hugo has
+processed it.)
+
+Here's another example, taken from [(org) LaTeX fragments](http://orgmode.org/manual/LaTeX-fragments.html).
+
+Below in Org:
+
+```text
+If $a^2=b$ and \( b=2 \), then the solution must be either
+$$ a=+\sqrt{2} $$ or \[ a=-\sqrt{2} \]
+```
+
+renders to:
 
 If \\(a^2=b\\) and \\( b=2 \\), then the solution must be either
-\\(a=+\sqrt{2}\\) or \\(a=-\sqrt{2}\\).
+\\[ a=+\sqrt{2} \\] or \\[ a=-\sqrt{2} \\]
+
+(Note that the last two equations show up on their own lines.)

ox-blackfriday.el

@@ -109,8 +109,11 @@ INFO is a plist holding contextual information."
      ((memq processing-type '(t mathjax))
       (let* ((frag (org-html-format-latex latex-frag 'mathjax info))
              ;; https://gohugo.io/content-management/formats#solution
-             (frag (replace-regexp-in-string "\\(\\\\[()]\\)" "\\\\\\1" frag)) ;\( -> \\(, \) -> \\)
-             (frag (replace-regexp-in-string "_" "\\\\_" frag))) ;_ -> \_
+             (frag (replace-regexp-in-string "_" "\\\\_" frag)) ;_ -> \_
+             ;; Need to escape the backslash in "\(", "\)", .. to
+             ;; make Blackfriday happy. So \( -> \\(, \) -> \\),
+             ;; \[ -> \\[ and \] -> \\].
+             (frag (replace-regexp-in-string "\\(\\\\[]()[]\\)" "\\\\\\1" frag)))
         frag))
      ((assq processing-type org-preview-latex-process-alist)
       (let ((formula-link