how to include a sub tex file in include file of latex

Question

I already include file in main.tex like this:

\documentclass{book}

\begin{document}
\include{chapter1}
\end{document}

how to include a subfile in chapter1.tex like this(or recursive include):

\chapter{chapter1}

\include{subsection}

Answer 1

In your main.tex, you can use \include{} or \input{}, see this question about their difference. Because you cannot use the \include in an nested way, you have to use \input in files that are already included (or inputed) themselves.

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So you have to change the \include to an \input in your chapeter1.tex.