Add support for Kramdown's math mode $$ ... $$.

_includes/mathjax.html

@@ -0,0 +1,36 @@
+<script type="text/x-mathjax-config">
+  MathJax.Hub.Config({
+    extensions: [
+      "MathMenu.js",
+      "MathZoom.js",
+      "AssistiveMML.js",
+      "a11y/accessibility-menu.js"
+    ],
+    jax: ["input/TeX", "output/CommonHTML"],
+    TeX: {
+      extensions: [
+        "AMSmath.js",
+        "AMSsymbols.js",
+        "noErrors.js",
+        "noUndefined.js",
+      ]
+    }
+  });
+  MathJax.Hub.Register.StartupHook("TeX Jax Ready",function () {
+    var TEX = MathJax.InputJax.TeX;
+    var COLS = function (W) {
+      var WW = [];
+      for (var i = 0, m = W.length; i < m; i++)
+        {WW[i] = TEX.Parse.prototype.Em(W[i])}
+      return WW.join(" ");
+    };
+    TEX.Definitions.Add({
+      environment: {
+        psmallmatrix: ['Array',null,'(',')','c',COLS([1/3]),".2em",'S',1],
+      }
+    });
+  });
+</script>
+<script type="text/javascript" async
+  src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/latest.js">
+</script>

_layouts/default.html

@@ -51,5 +51,6 @@ <h1 class="site-name"><a href="{{ site.baseurl }}/">{{ site.name | markdownify }
 
     {% include analytics.html %}
     {% include anchorjs.html %}
+    {% include mathjax.html %}
   </body>
 </html>