下面是关于函数f（x）=|x﹣{x}|的四个命题：①函数y=f（x）的定义域是R，值域是 $0,\frac{1}{2}$ ；②函数y=f（x）的图象关于直线 $x=\frac{k}{2}\left(k\in Z\right)$ 对称；③函数y=f（x）是周期函数，最小正周期是1；其中正确的命题序号是____．

阅读下面文段回答问题。

        I'm Mrs Zhou. I'm a teacher in No. 1 Middle School(第一中学). This is Li Lei. He's Chinese. He is in Class 2, Grade 7. He's my student, and my son(儿子)too. He's twelve.

       That girl is Jean. She's American. She's eleven. And that boy is Jack. He's English. He's ten. They're in Li Lei's class.

I'm their Chinese teacher. All the students and the teachers have two names: an English name and a Chinese name. Li Lei's English name is Tom. My English name is Mary. Their English teacher Miss Jin's English name is Sue. Jean's and Jack's Chinese names are Wei Hua and Zhang Li.

一个人站在南极点上，他的前后左右都是（  ） 

历史学家乔科尔顿写道：“这场战争前后持续了四年，其中有好些像拿破仑时代一样的大规模战斗，这是除了中国1853—1864年的太平天国起义之外，在19世纪发生的最惨烈的战争。”此战争应该是（  ）