space after a comma in a subscript?

Question

I have a problem with a space after a comma in a subscript. I created a command called \segN, for intervals with integers. I want a \thinspace between the first integer and the comma, so I typed #1\,,#2 in my command. The second integer should be separated from the comma by the regular space after a comma. It works fine but not in a subscript. Here is a MWE:

\documentclass[a4paper,10pt]{article}

\usepackage{mathtools}
\usepackage[frenchmath]{newtxmath}

\makeatletter
\newcommand{\segN}{\@ifstar\segN@star\segN@nostar}
\newcommand*{\segN@star}[2]{\left\dlb#1\,,#2\right\drb}
\newcommand*{\segN@nostar}[3][]{#1\dlb#2\,,#3#1\drb}
\makeatother

\begin{document}
  $\segN{1}{2}$,\qquad$\displaystyle\sum_{k\in\segN{1}{2}}x_k$
\end{document}

In the first \segN{1}{2}, it's ok: the 2 is separated from the comma. But in the second (with the subscript), the 2 is sticked to the comma and I don't understand why.

Answer 1

The space between a punctuation symbol and an ordinary symbol is a "conditional" \thinmuskip: "conditional" in the sense that it's not used in script styles: that's why you see it in the first example but not in the second one. (See this answer for more details.)

Quick fix: make everything a \mathord and insert both spaces manually.

\documentclass[a4paper,10pt]{article}

\usepackage{mathtools}
\usepackage{newtxtext}
\usepackage[frenchmath]{newtxmath}

\DeclarePairedDelimiterX{\segN}[2]{\dlb}{\drb}{{#1}\,{,}\,{#2}}

% also a possibility but more complicated
%\DeclarePairedDelimiterX{\segN}[2]{\dlb}{\drb}{#1\,,\,\nonscript\!#2}

\begin{document}
  $\segN{1}{2}$,\qquad$\displaystyle\sum_{k\in\segN{1}{2}}x_k$
\end{document}

Miscellaneous comments:

• mathtools already provides a way to define a paired delimiter with a starred version.
• Loading newtxmath without newtxtext looks weird to me.

Answer 2

Is it what You need?

\documentclass[a4paper,10pt]{article}

\usepackage{mathtools}
\usepackage[frenchmath]{newtxmath}

\makeatletter
\newcommand{\segN}{\@ifstar\segN@star\segN@nostar}
\newcommand*{\segN@star}[2]{\left\dlb#1\,,#2\right\drb}
\newcommand*{\segN@nostar}[3][]{#1\dlb#2, \,#3#1\drb}
\makeatother

\begin{document}
  $\segN{1}{2}$,\qquad$\displaystyle\sum_{k\in\segN{1}{2}}x_k$
\end{document}