一个简单的例子：
\documentclass[a4paper,12pt,notitlepage]{article}
\usepackage[warn]{mathtext}
\usepackage[T1,T2A]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[russian,english]{babel}
\begin{document}
Например $А_б$.
\end{document}
日志文件，在清除 Basic MiKTeX 安装后首次运行示例，没有任何用户格式等：
This is pdfTeX, Version 3.14159265-2.6-1.40.19 (MiKTeX 2.9.6630 64-bit) (preloaded format=pdflatex 2018.6.1) 1 JUN 2018 11:44
entering extended mode
**./test18.tex
(test18.tex
LaTeX2e <2017-04-15>
Babel <3.18> and hyphenation patterns for 75 language(s) loaded.
("C:\Program Files\MiKTeX 2.9\tex\latex\base\article.cls"
Document Class: article 2014/09/29 v1.4h Standard LaTeX document class
("C:\Program Files\MiKTeX 2.9\tex\latex\base\size12.clo"
File: size12.clo 2014/09/29 v1.4h Standard LaTeX file (size option)
)
\c@part=\count79
\c@section=\count80
\c@subsection=\count81
\c@subsubsection=\count82
\c@paragraph=\count83
\c@subparagraph=\count84
\c@figure=\count85
\c@table=\count86
\abovecaptionskip=\skip41
\belowcaptionskip=\skip42
\bibindent=\dimen102
)
(C:\Users\vfkom\AppData\Roaming\MiKTeX\2.9\tex\latex\t2\mathtext.sty
Package: mathtext 2018/04/13 v1.1 transparent text-and-math defs
LaTeX Info: Redefining \halign on input line 119.
)
("C:\Program Files\MiKTeX 2.9\tex\latex\base\fontenc.sty"
Package: fontenc 2017/04/05 v2.0i Standard LaTeX package
("C:\Program Files\MiKTeX 2.9\tex\latex\base\t1enc.def"
File: t1enc.def 2017/04/05 v2.0i Standard LaTeX file
LaTeX Font Info: Redeclaring font encoding T1 on input line 48.
\symT1letters=\mathgroup4
)
("C:\Program Files\MiKTeX 2.9\tex\latex\cyrillic\t2aenc.def"
File: t2aenc.def 2005/09/27 v1.0i Cyrillic encoding definition file
Now handling font encoding T2A ...
... processing UTF-8 mapping file for font encoding T2A
("C:\Program Files\MiKTeX 2.9\tex\latex\base\t2aenc.dfu"
File: t2aenc.dfu 2017/01/28 v1.1t UTF-8 support for inputenc
! Undefined control sequence.
l.46 \DeclareUnicodeCharacter
{00A4}{\textcurrency}
The control sequence at the end of the top line
of your error message was never \def'ed. If you have
misspelled it (e.g., `\hobx'), type `I' and the correct
spelling (e.g., `I\hbox'). Otherwise just continue,
and I'll forget about whatever was undefined.
! LaTeX Error: Missing \begin{document}.
See the LaTeX manual or LaTeX Companion for explanation.
Type H <return> for immediate help.
...
l.46 \DeclareUnicodeCharacter{0
0A4}{\textcurrency}
You're in trouble here. Try typing <return> to proceed.
If that doesn't work, type X <return> to quit.
! Undefined control sequence.
l.46 \DeclareUnicodeCharacter{00A4}{\textcurrency
}
The control sequence at the end of the top line
of your error message was never \def'ed. If you have
misspelled it (e.g., `\hobx'), type `I' and the correct
spelling (e.g., `I\hbox'). Otherwise just continue,
and I'll forget about whatever was undefined.
! Undefined control sequence.
l.47 \DeclareUnicodeCharacter
{00A7}{\textsection}
The control sequence at the end of the top line
of your error message was never \def'ed. If you have
misspelled it (e.g., `\hobx'), type `I' and the correct
spelling (e.g., `I\hbox'). Otherwise just continue,
and I'll forget about whatever was undefined.
! TeX capacity exceeded, sorry [input stack size=5000].
\textsection ->\T1-tmcmd \textsection
\T1\textsection \T1math\textsection
l.47 \DeclareUnicodeCharacter{00A7}{\textsection
}
If you really absolutely need more capacity,
you can ask a wizard to enlarge me.
Here is how much of TeX's memory you used:
495 strings out of 493305
5269 string characters out of 3138875
54363 words of memory out of 3000000
4120 multiletter control sequences out of 15000+200000
3938 words of font info for 15 fonts, out of 3000000 for 9000
1141 hyphenation exceptions out of 8191
5000i,0n,27p,163b,36s stack positions out of 5000i,500n,10000p,200000b,50000s
! ==> Fatal error occurred, no output PDF file produced!
答案1
在再次启动（重新设计的）MiKTeX 控制台并进行更新和刷新后，该问题已得到解决（问题原因尚不清楚）。感谢所有关注的人。