There is an old Chinese saying, “If someone gives you a little drop of water, you should return a spring of water." That means if someone helps you a bit, you should remember it and thank them gratefully(感激的）in return.

       We should live in a world of gratitude. We should prove it with actions, not just words. We thank people who hurt us, because they teach us important things. We thank people who give us up, because we learn how to be independent. We thank people who selfish(自私的）, because we learn the importance of being kind. Of course, we should thank people who help us. We should thank people who teach us. We should thank people who care about us.

      We thank our parents. They work hard all the time in order to give their sons and daughters a better life. They cook meals for us, they do the chores for us, they always give us food and clothes, they teach us what is right and what is wrong.

      We thank our teachers who give us dreams. They teach us a lot of things but they ask for nothing from us. We are well-educated of them.

     There are a lot of people that we should be grateful to in our life. We should find a way to thank them.

①已知随机变量X～N（0，σ²），若P（|X|＜2）=a，则P（X＞2）的值为 $\frac{1+a}{2}$ ；

②设a、b∈R，则“log₂a＞log₂b”是“2^a﹣b＞1”的充分不必要条件；

③函数f（x）= $x^{\frac{1}{2}}$ ﹣（ $\frac{1}{2}$ ）^x的零点个数为1；

④命题p：∀n∈N，3ⁿ≥n²+1，则￢p为∀n∈N，3ⁿ≤n²+1．

其中真命题的序号为____．