inconsistent compiling of bold symbols in LaTeX

Question

I am new to LaTeX and I am writing up an assignment and having trouble with bold, subscripted symbols. I want to write a product between a vector c_j and a scalar R_jk. I want the vector to be bold, and I want the scalar to be normal, so I write:

\textbf{$c_{k}$}$R_{kk}$

The R_kk here comes out bold, and I'm not sure why. Even if I end the sentence after the c_k and then write $R_{kk}$, it still comes out bold. Earlier in the assignment I have a term alpha - R_kk, both scalars, so I write:

$(\alpha - R_{kk})$

and this looks fine. My goal is to write c_k R_jk, next to each other to denote a scalar-vector product, and to have the c_k bold and the R_jk normal. Currently I can't figure out how. Any ideas?

Thanks for your time.

Answer 1

\textbf{$c_{k}$}$R_{kk}$

\textbf sets the text font to bold but has no effect on math within that scope. A bold math variable should be \mathbf so

\documentclass{article}
\begin{document}
$\mathbf{c}_k R_{kk}$
\end{document}