accessing controller variable from view

In my ApplicatoinController I have:

class ApplicationController < ActionController::Base

  MOBILE_USER_AGENTS = 'palm|palmos|palmsource|iphone|blackberry|
nokia>phone>midp>mobi>pda>' +
ericsson>lg>audiovox>motorola>' +
windows ce|smartphone|' +
portable>sprint>vodafone>' +
minimo>plucker>netfront>' +

  def is_mobile_device?
     request.user_agent.to_s.downcase =~

My problem is in my /views/layouts/application.html.erb I have:

<% if is_mobile_device? %>

I get:

undefined method `is_mobile_device?' for #<ActionView::Base:0x6752ff0>

How can I refer to is_mobile_device? from this view -- as well as all
views and controllers -- I want to be DRY about this - how can I do
that? -Janna B.

Lookup helper_method