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

Question

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).

Answer 1

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}