What is the title of the course that is a prerequisite for Mobile Computing?
SELECT title FROM course WHERE course_id IN (SELECT T1.prereq_id FROM prereq AS T1 JOIN course AS T2 ON T1.course_id  =  T2.course_id WHERE T2.title  =  'Mobile Computing')
prerequisite for a course refers to finding the course_id(s) that are pre-required (prereq_id) for a given course; title of the course is obtained by selecting title from the course table where course_id matches the prerequisite course_id;