A simple liquid tag for Jekyll that converts {% m %} and {% em %} into inline math, and {% math %} and {% endmath %} into block equations, by replacing with the appropriate MathJax script tags.

Jekyll Liquid tag for MathJax.rb

module Jekyll
  class MathJaxBlockTag < Liquid::Tag
    def render(context)
      '<script type="math/tex; mode=display">'
    end
  end
class MathJaxInlineTag < Liquid::Tag
    def render(context)
      '<script type="math/tex">'
    end
  end
class MathJaxEndTag < Liquid::Tag
    def render(context)
      '</script>'
    end
  end
end

Liquid::Template.register_tag('math', Jekyll::MathJaxBlockTag)
Liquid::Template.register_tag('m', Jekyll::MathJaxInlineTag)
Liquid::Template.register_tag('endmath', Jekyll::MathJaxEndTag)
Liquid::Template.register_tag('em', Jekyll::MathJaxEndTag)