# Why does this throw an error for double subscripting?

{\mathbf{\hat{\Sigma}_{\mathrm{hom}}}}_{jj}^{1/2}


So I thought if you enclosed the term with parentheses, you can do a double subscript, but this throws a double subscript error, and I'm not sure why. My guess is that the hat command is interfering but I don't know why that's the case.

• Please clarify what the intended result is supposed to be. E.g., are jj and 1/2 supposed to be subscripts and superscripts to (a) \mathrm{\mathrm{hom}} or (b) \hat{\boldsymbol{\Sigma}}_{\mathrm{hom}}? – Mico Jan 29 at 7:39
• The latter, I thought enclosing it with parentheses is enough. – Sky Jan 29 at 7:43
• Under normal circumstances you would be right (I mean enclosing with parentheses). But... there is a but: when the content of the math list within {...} is a single accent atom (with or without a sub- or superscript), the grouping is removed. That's what causes the error here. See Why am I getting a double subscript error? – campa Jan 29 at 8:19

I'm listing two possible solutions. In the first solution, I employ round parentheses to provide some visual clues for readers.

\documentclass{article}
\usepackage{amsmath}  % for \text directive
\usepackage{bm}       % for \bm directive (\bm more robust than \boldsymbol)

\begin{document}
$(\widehat{\bm{\Sigma}}_{\hom})_{jj}^{1/2}$
$\widehat{\bm{\Sigma}}_{\hom}{}_{jj}^{1/2}$
\end{document}

• bm is more robust: \usepackage{amsmath,bm} and \bm{\Sigma} (but \boldsymbol becomes an alias for \bm). – egreg Jan 29 at 10:55
• @egreg - Many thanks for your suggestion; I've incorporated it in the answer – Mico Jan 29 at 11:22

Don't use \mathbf for Greek letters, hope the below code may meet your expectation:

\documentclass{book}
\usepackage{mathtools}
\begin{document}

$\boldsymbol{\hat{\Sigma}}_{{\mathrm{hom}}_{jj}^{1/2}}$

\end{document}


• I had the answer \documentclass[a4paper,12pt]{article} \usepackage{amsmath,amssymb} \begin{document} $\mathbf{\hat{\sum}}_{{\mathrm{hom}_{jj}}^{1/2}}$ \end{document} :-) +1 – Sebastiano Jan 29 at 7:29