non-trivial:
https://www.zhihu.com/question/277461723/answer/394556426
https://www.zhihu.com/question/20681622/answer/18313127
asymptotically tight:
https://blog.csdn.net/weixin_30795127/article/details/94817650
https://blog.csdn.net/weixin_34372728/article/details/85513825
https://zhuanlan.zhihu.com/p/40151536
