For Problem 13 and 14: Let X1 be an integral variable (not binary!). Let Y1 be a binary variable, i.e., Y1 can be 0 or 1 only. 13. Write a constraint to represent "If X1=0 then Y1=0, and if X1>0 then Y1 can be either 0 or 1". 14. Write a constraint to represent "If X1>0 then Y1=1, and if X1=0 then Y1 can be either 0 or I". Note: The constraints in question 13 and 14 together indicate such a relationship between variables X1 and Y1 : If X1 is zero, then Y1 must be rero; and if X1 is not zero, then Y1 must be 1. Such a relation is what we want to formulate in the "fixed charge