protect the math environments for epub as suggested in jgm/pandoc#2758 …

…(not an optimal solution, and hoping Pandoc can address the real issue)

R/ebook.R

@@ -60,6 +60,7 @@ epub_book = function(
       )
       content = restore_part_epub(content)
       content = restore_appendix_epub(content)
+      content = protect_math_env(content)
       writeUTF8(content, input_file)
       NULL
     },
@@ -120,6 +121,23 @@ restore_appendix_epub = function(x) {
   x
 }
 
+# wrap math environments in $$, otherwise they are discarded by Pandoc
+# https://github.com/jgm/pandoc/issues/2758
+protect_math_env = function(x) {
+  env = 'equation'  # may add more LaTeX environments later
+  s1 = sprintf('\\begin{%s}', env)
+  s2 = sprintf('\\end{%s}', env)
+  for (s in s1) {
+    i = x == s
+    x[i] = paste0('$$', x[i])
+  }
+  for (s in s2) {
+    i = x == s
+    x[i] = paste0(x[i], '$$')
+  }
+  x
+}
+
 # manually base64 encode images in css: https://github.com/jgm/pandoc/issues/2733
 epub_css = function(files, output = tempfile('epub', fileext = '.css')) {
   css = unlist(lapply(files, function(css) {