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I must define number be for subsubsection form :

\chapter : 1

\section : 1.1

\subsection : 1.1.1

\subsubsection : 1.1.1.a

Can you help me define form.

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The numbering of subsections in the book document class should already be in the format you require. To modify \thesubsubsection to show the final "number" in alphabetical rather than arabic-numeral format, you could issue the commands

\makeatletter
\renewcommand\thesubsubsection{\thesubsection.\@alph\c@subsubsection}
\makeatother

in the preamble of your document.

As you can probably tell from this definition, \c@subsubsection is a counter variable, \@alph instructs TeX to display the value of the counter in lowercase-alphabetical format (a, b, c, etc), and \thesubsection. instructs TeX to prepend the (already-defined) representation of the subsection counter, followed by a dot (.), to \@alph\c@subsubsection.


As @ThorstenDonig has pointed out in a comment, the preceding commands -- which are fairly low-level -- can be replaced with the following, higher-level command (which is especially nice at it obviates the need to use \makeatletter and \makeatother):

\renewcommand\thesubsubsection{\thesubsection.\alph{subsubsection}}
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    Why so complicated? \renewcommand{\thesubsubsection}{\thesubsection.\alph{subsubsection}} doesn't need \makeatletter and \makeatother. – Thorsten Donig Nov 22 '13 at 14:38
  • I concur with Thorsten: the user level command \alph is there just for this purpose. – egreg Nov 22 '13 at 15:40
  • @ThorstenDonig - Because \alph{<counter>} is defined in terms of \@alph and \c@<counter> and because I like using low-level commands... – Mico Nov 22 '13 at 16:13
  • Low level commands are for the advanced user. For beginners they are only confusing. And I don't think that the "normal" user is interested in low level commands. – Thorsten Donig Nov 22 '13 at 17:31
  • @ThorstenDonig - Feel free to downvote my answer. – Mico Nov 22 '13 at 18:09

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