In the set X=a,b,c,d, which of the following functions in X ?
R1={(b,a)(a,b),(c,d),(a,c)}
R2={(a,d)(d,c),(b,b),(c,c)}
R3={(a,b)(b,c),(c,d),(b,d)}
R4={(a,a)(b,b),(c,c),(a,d)}
X={a,b,c,d}R1=b,a,a,b,c,d,a,c a,b∈R1 and a,c∈R, ∴R1 is not a function. R2={a,d,d,c,b,b,c,c}Hence, R2 is a function.