Cannot stop Latin character being emboldened after Greek character emboldened

Question

I am using the following code.

\documentclass[]{article}
\usepackage{amssymb}
\usepackage{amsmath}

\begin{document}
    
    \begin{eqnarray}  \label{eq:linearInterpolant}
        \boldsymbol{\beta}(\bf{s^*}) = \normalfont{c}
    \end{eqnarray}
    
\end{document}

I get the following result

That is, the c character is emboldened despite the boldface have been ended before the c. It seems I should not need the \normalfont but it doesn't work anyway. I would like the c to be non-bold as it is does not represent a vector.

Answer 1

There are two distinct issues with your code:

• \bf is a switch and doesn't take an argument. Its scope extends to the end of the current group. (Same for \normalfont, which incidentally is a text-mode command and has no effect when used in math mode.) Hence

  (\bf{s^*}) = \normalfont{c}
  

  works out to be the same as

  (\bf s^*) = c
  

  c is rendered in bold because the scope of \bf includes it.

• \bf is a hold-over from Plain TeX and shouldn't be used in LaTeX documents created after 1994 [!]. Really. \bf and its close relatives, such as \rm and \it, are no longer defined in the LaTeX kernel. Some document classes still define \bf, but others do not. To maximize the portability of your documents across document classes, you should replace all instances of {\bf ...} with either \textbf{...} or \mathbf{...}, depending on whether text mode or math mode is in use.

In short, you should learn to replace

\boldsymbol{\beta}(\bf{s^*}) = \normalfont{c}

with

\boldsymbol{\beta}(\mathbf{s}^*) = c

and live happily ever after. (Observe that I would only make s bold, but not the superscript asterisk.)