# Is there a bold italic \sigma and a sans serif bold italic \sigma?

And they look differently?

I'm trying to typeset iso conform (vectors bold italic and tensors sans serif bold italic).

Mechanical stress $\sigma$ is a tensor, but I also want to typeset the normal stress, which is the tensor multiplied by the unit normal $\mathbf n$ and hence, a vector as well.

I tried bm (results in serif bold \sigma) or isomath (error font shape OML/ptm/bx/it undefined).

-
Is \usepackage{bm}...\begin{document}$\bm\sigma$ what you want? Or how exactly do you expect \sigma to appear? – zeroth Mar 14 '13 at 11:31
I found this on wikipedia: 𝝈 (mathematical bold italic small sigma) and 𝞼 (mathematical sans-serif bold italic small sigma). $\bm \sigma$ yields a very bold σ, but I don't know what it is exactely. – Sebastian Mar 14 '13 at 11:59
The comprehensive reference tex.ac.uk/tex-archive/info/symbols/comprehensive/symbols-a4.pdf is a very useful resource that has many varieties of greek symbols, including bold. Whether there is one that fits your need is to be determined. – Steven B. Segletes Mar 14 '13 at 12:14
the comprehensive reference lists \upsigma (serif, upright), but no sans-serif. – Sebastian Mar 14 '13 at 13:44
I think the answer offered by @andrew is very nice, but I'm still a bit reluctant to switch all my documents to lualatex (no stable release yet). – Sebastian Mar 14 '13 at 13:47

If you have the option of using lualatex then unicode-math provides access to such characters:

\documentclass{article}

\usepackage{luatextra,lualatex-math}
\usepackage[math-style=ISO,sans-style=italic]{unicode-math}
\defaultfontfeatures{Ligatures=TeX}
\setmainfont{Tex Gyre Termes}
\setsansfont{Tex Gyre Heros}
\setmonofont{Tex Gyre Cursor}
\setmathfont{Tex Gyre Termes Math}

\begin{document}
$$\sigma \mathbf{\sigma} \mathbfsf{\sigma}$$

$$\psi \mathbf{\psi} \mathbfsf{\psi}$$
\end{document}


Tex Gyre Termes Math is quite new, an older font is Tex Gyre Pagella Math, the name has just changed from TG Pagella Math:

\documentclass{article}

\usepackage{luatextra,lualatex-math}
\usepackage[math-style=ISO,sans-style=italic]{unicode-math}
\defaultfontfeatures{Ligatures=TeX}
\setmainfont{Tex Gyre Pagella}
\setsansfont{Tex Gyre Heros}
\setmonofont{Tex Gyre Cursor}
\setmathfont{Tex Gyre Pagella Math}

\begin{document}
$$\sigma \mathbf{\sigma} \mathbfsf{\sigma}$$

$$\psi \mathbf{\psi} \mathbfsf{\psi}$$
\end{document}


Alternatively there are the XITS fonts, where the distinction is less clear:

\documentclass{article}

\usepackage{luatextra,lualatex-math}
\usepackage[math-style=ISO,sans-style=italic]{unicode-math}
\defaultfontfeatures{Ligatures=TeX}
\setmainfont{XITS}
\setmathfont{XITS Math}

\begin{document}
$$\sigma \mathbf{\sigma} \mathbfsf{\sigma}$$

$$\psi \mathbf{\psi} \mathbfsf{\psi}$$
\end{document}

-
Thanks for your answer. I tried compiling this with lualatex, but for this I'd need to install some more fonts. I'm considering doing this in the future. – Sebastian Mar 14 '13 at 12:25
I have added a couple of older font suggestions. All the fonts are in the latest texlive. – Andrew Swann Mar 14 '13 at 12:47
thanks!!! using \setmathfont{TG Pagella Math} works in my latex environment!! – Sebastian Mar 14 '13 at 13:29