UCSD criteria to declare math or economics if you enter undeclared:
https://www.math.ucsd.edu/~handbook/undergraduate/cappedstatus/
https://economics.ucsd.edu/undergraduate-program/entry-to-the-majors.html
UCLA criteria to declare math or economics:
https://www.math.ucla.edu/ugrad/majors/math-major
http://www.math.ucla.edu/ugrad/majors/mathecon
https://economics.ucla.edu/undergraduate/current-students/majors-and-minors/economics-b-a/