y1(t): number of students studying option 1
y2(t): number of students studying option 2
: input of fresh students per time interval
a: effectiveness of communication between the students y1 and y2
: backflow from y2 to y1 ( )
d1: rate of successful final examination of y1 (d1=0.1)
d2: rate of successful final examination of y2 (d2=0.3)
: portion of graduate students ( )
mapping: