Theorem 3.6: Let g be an everywhere function from A to B, and f be an everywhere function from b to c. Then &( fand g are onto, then fo g is also onto 4(2)Iff and g are one to one, then fog is also one to one &3)Iffand g are one-to-one correspondence, then fog is also one-to-one correspondence 令 Proof:(1) for every c∈C, there exists a∈ A such that f°g(a)=c❖ Theorem 3.6: Let g be an everywhere function from A to B, and f be an everywhere function from B to C. Then ❖ (1)if f and g are onto , then f  g is also onto. ❖ (2)If f and g are one to one, then f  g is also one to one. ❖ (3)If f and g are one-to-one correspondence, then f  g is also one-to-one correspondence ❖ Proof: (1) for every cC, there exists aA such that f  g(a)=c