Proof.")": Supposing r 2 X X while r 2 lm. Because r 2 X
X,r = 0 and r = 0; Because r 2 lm, m r = 0. So, m = 0 and
m = 0, which means 2 lm and 2 lm.
"(": Because 2 lm, m 2 X; in the same argument, m 2 X. As a
result, m 2 X X which means X X 6= ;.
Finally, in summary, we get the theorem.