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Here is my code:

 \documentclass[12pt,a4paper]{book} 
 \begin{document}
 \textbf{\textit{Les caractéristiques $f_i$}}
 \end{document}

enter image description here

My question is: why "fi" isn't in bold ?

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8  
Please, do not do this. Bolding math in bold environment is a very bad typographical practice, since bold and non-bold f can be (and quite often are) two completely different things. For the same reason, \sin stays upright even in theorem body, etc. –  yo' Dec 10 '13 at 16:13

4 Answers 4

up vote 1 down vote accepted

There is also \boldsymbol but it does not work so well as the above answers. See MWE below:

enter image description here

\documentclass[letterpaper]{article}
\usepackage{amsmath,amssymb}
\begin{document}
\noindent
\textbf{\textit{Les caractéristiques $\boldsymbol f_{\!\boldsymbol i}$}}\\
\textbf{\boldmath\textit{Les caractéristiques $f_i$}}
\end{document}
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It works with \boldmath . Thanks. –  researcher Dec 10 '13 at 15:43
5  
Bolding math in bold environment is a very bad typographical practice, since bold and non-bold f can be (and quite often are) two completely different things. For the same reason, \sin stays upright even in theorem body, etc. –  yo' Dec 10 '13 at 16:00
    
True, true @tohecz. I am just throwing out ideas that I had seen some months aback in code that I borrowed from friend. –  azetina Dec 10 '13 at 16:07
2  
@azetina And I'm just adding a disclaimer to warn people that even though they ask for it, they actually don't want it ;) –  yo' Dec 10 '13 at 16:12
    
@tohecz Do edit my question and bold the disclaimer :-) –  azetina Dec 10 '13 at 16:13

The f isn’t bold because it’s set in math mode and in math f and f can be different things. E.g. f is a vector and f the length of the vector.

LaTeX has it’s own treatment for math fonts which can be affected by the user with commands like \mathbf. But the math fonts don’t depend on the surrounding text font. You may have a look at Make mathfont respect the surrounding family, where you can find settings to get your desired behaviour. An image taken frome there to illustrate the result:

result

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Please post complete documents not just fragments (so this isn't tested) but you want

 \begin{document}
 \textbf{\boldmath\textit{Les caractéristiques $f_i$}}
 \end{document}
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Just to mention all possibilities, there is also the bm package with its \bm command:

\documentclass[12pt,a4paper]{book}
\usepackage[utf8]{inputenc}
\usepackage{bm}
\begin{document}
\textbf{\textit{Les caractéristiques $\bm{f_i}$}}
\end{document} 

enter image description here

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