Find the title of course whose prerequisite is course Differential Geometry.
SELECT title FROM course WHERE course_id IN (SELECT T1.course_id FROM prereq AS T1 JOIN course AS T2 ON T1.prereq_id  =  T2.course_id WHERE T2.title  =  'Differential Geometry')
title of course whose prerequisite is a specific course refers to a subquery where the prerequisite course title matches a given title, which in this case is 'Differential Geometry'; the matching is done by joining the prerequisite table with the course table on their course identifiers.