Please I need a help!

@BarryLiu1995 can u check #1672? Mb, it can help.

@ivan-nginx This is not the problem

@BarryLiu1995 may be u right, but anyway i think what problem is not in Next theme.
Theme Next - it's just templates and styles with some javascripts engines.
Mathjax - it's third party javascript and not Next maintenance it. And as markdown engine too.
It seems, problem in markdown engine in compilation with mathjax or any third-party plugins, what directly support markdown and/or Hexo. And i really don't know about mathjax and how it work, but u may also try to research in our issues by this link or ask in MathJax | Markdown | NodeJS issues. Next it's just template, u must know and understand it.

@ivan-nginx Ok, I got it. I will try to fix my problem by myself.

@BarryLiu1995 i don't know what this is, but see - it's working:
In _config.yml:

# MathJax Support
mathjax:
  enable: true
  per_page: false
  cdn: //cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.1/MathJax.js?config=TeX-AMS-MML_HTMLorMML

In *.md file:

## MathJax fast test

$$lim_{1\to+\infty}P(|\frac{1}{n}\sum_i^nX_i-\mu|<\epsilon)=1, i=1,...,n$$

@ivan-nginx I change Front-matter like this and it work

这个帖子里有提到解决方法 (This post has a solution)

try to replace default with
//cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.1/MathJax.js?config=TeX-AMS-MML_HTMLorMML

最后的代码是 (final code)

mathjax:
  enable: true
  per_page: true
  #cdn: //cdn.bootcss.com/mathjax/2.7.1/latest.js?config=TeX-AMS-MML_HTMLorMML
  cdn: //cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.1/MathJax.js?config=TeX-AMS-MML_HTMLorMML