Two upper-level electives in mathematics and two in economics plus the senior requirement comprise the thematic focus dimension. "Upper-level" in economics means that the course has one or both Intermediate Theory courses as prerequisite. "Upper-level" in mathematics means simply those specialized courses above the 2000-level so designated by the Mathematics department. (As an exception, MATH UN1103 Intro to Number Theory and MATH E1210 Ordinary Differential Equations are approved electives.)
It's a good idea to plan ahead, which means identifying the electives you want and organizing your common core sequence in order to have the appropriate prerequisite at the right time. It also means you should have completed the calculus sequence followed by linear algebra and analysis and optimization.
The primary purpose of the upper-level electives requirement is to solidify your understanding of the common core concepts and tools by applying them to some specific set of problems or issues. A secondary purpose is to provide you with background in a particular area on which you can subsequently build in your senior thesis or seminar.
The senior requirement comprises three different options:
1. one-semester economics seminar option (ECON BC3063);
2. one-semester economics thesis option (ECON BC3062), although two semesters are optional;
3. one semester of undergraduate seminar in mathematics (MATH UN3951 or UN3592).