假定英语课上老师要求同桌之间交换修改作文，请你修改你同桌写的以下作文。文中共有10处语言错误，每句中最多有两处。错误涉及一个单词的增加、删除或修改。
增加：在缺词处加一个漏字符号（∧），并在其下面写出该加的词。
删除：把多余的词用斜线（/）划掉。
修改：在错的词下划一横线，并在该词下面写出修改后的词。
注意：1）每处错误及其修改均仅限一词；
2）只允许修改10处，多者（从第11处起）不计分。
Dear Mary,
I receive your E-mail just now. Don't worry about me. I'm getting on well with my research work in the lab. But to my greatly surprise you say you will give up learn English. The reason is because you have not done well in it recently and you have lost interests. I'm afraid I couldn't agree with you . I know it is difficulty to learn English, and English is widely used in the world today. It will be important tool in our future work. Beside, it is becoming more and more important in our daily life. If you study hard, you will be succeed. Do remember that where there is a will, there is a way. I' m looking forward to hearing good news from you.
Yours,
Li Hua
,-1.1=-2，定义函数{x}=x-x，给出下列四个
有无数解；③函数{x}是周期函数;④函数{x}是增函数.其中正确的命题序号有 （ ）
