# \bf in the first argument of \cdot; causes the whole expression to become bold

I want just the first argument in bold, but when i try to select just the first argument using the \bf command it modified the whole expression. This is what I had

$a = \frac12 \cdot \frac13 + \frac14 \\$


I just wanted \frac12 in bold so I used

$a = \bf{\frac12} \cdot \frac13 + \frac14 \\$


But this modified all the expression in bold.

Any clue how to solve this?

• \bf is deprecated for more than twenty years now – user31729 Sep 16 '14 at 20:39
• Welcome to TeX.SX! Please help us to help you and add a minimal working example (MWE) that illustrates your problem. It will be much easier for us to reproduce your situation and find out what the issue is when we see compilable code, starting with \documentclass{...} and ending with \end{document}. – user31729 Sep 16 '14 at 20:40
• Do you mean \frac{1}{2} instead of \frac12? – user31729 Sep 16 '14 at 20:41
• @ChristianHupfer \frac12 is short for \frac{1}{2}, because argument braces can be dropped, if the argument consists of one token only. – Heiko Oberdiek Sep 16 '14 at 20:43
• @HeikoOberdiek: I know that, but's no nice use of the command ;-) – user31729 Sep 16 '14 at 20:43

\bf is deprecated since LaTeX2e. Now \bfseries should be used in text mode. Both are commands that never take an argument, they remain active until the end of the current group or overwritten by another font switch command.
In math mode there are other options. Here \mathbf will do:
\documentclass{article}
$a = \mathbf{\frac12} \cdot \frac13 + \frac14$

• @JuanMuñoz \bfseries is the switch equivalent of \bf. If you want a limited bold face effect you plugin as an argument to \textbf{This text is bold.} This is not – percusse Sep 16 '14 at 21:56