Bracket covers sum limits using mathclap environment

If you don't use \mathclap you can add negative space to close up the summand, but the problem is made worse occurs as \left makes over-size delimiters, there is no need to make the delimiters cover the limits here,

\documentclass{article} \usepackage{mathtools} \begin{document} \[S_1 = \frac{1}{2} \left(\;\sum_{\mathclap{i = n + 1}}^{n + m} a_i t_i \right)\] \[S_1 = \frac{1}{2} \left(\sum_{i = n + 1}^{n + m}\! a_i t_i \right)\] \[S_1 = \frac{1}{2} \Bigl(\,\sum_{\mathclap{i = n + 1}}^{n + m} a_i t_i \Bigr)\] \end{document} 

That's expected, because \mathclap makes a zero width box, so TeX cannot “see” the width of those limits.

I wouldn't use \left and \right, which produce too big delimiters in this case, but \biggl and \biggr, with a little adjustment.

Here I present a few realizations.

• (1) is clearly wrong;
• (2) is also wrong because of the too large size and the unwanted space between the fraction and the parenthesis;
• (3) is better as regards to size and spacing, but the parenthesis clashes with the subscript;
• (4) fixes the clash;
• (5) fixes the excess space between the summation and the terms to sum;
• (6) does the same as (5), but the backing up is too much, in my opinion.

Take your pick. Remember that automation and fine typography don't go along well.

\documentclass{article} \usepackage{mathtools} \begin{document} \begin{alignat}{2} S_1 &= \frac{1}{2} \left(\sum_{\mathclap{i = n + 1}}^{n + m} a_i t_i \right) &\qquad& \text{your attempt} \\ S_1 &= \frac{1}{2} \left(\sum_{i = n + 1}^{n + m} a_i t_i \right) &\qquad& \text{without \texttt{\string\mathclap}} \\ S_1 &= \frac{1}{2} \biggl(\sum_{i = n + 1}^{n + m} a_i t_i \biggr) && \text{with \texttt{\string\bigg}} \\ S_1 &= \frac{1}{2} \biggl(\,\sum_{i = n + 1}^{n + m} a_i t_i \biggr) && \text{with the needed correction} \\ S_1 &= \frac{1}{2} \biggl(\,\sum_{i = n + 1}^{n + m} \hspace{-0.33em} a_i t_i \biggr) && \text{with some back space} \\ S_1 &= \frac{1}{2} \biggl(\,\smashoperator[r]{\sum_{i = n + 1}^{n + m}} a_i t_i \biggr) && \text{with \texttt{\string\smashoperator}} \end{alignat} \end{document} 

You may want \smashoperator also provided by mathtools?

\documentclass{article} \usepackage{mathtools} \begin{document} \[S_1 = \frac{1}{2} \left(\sum_{i = n + 1}^{n + m} a_i t_i \right)\] \[S_1 = \frac{1}{2} \left(\sum_{\mathclap{i = n + 1}}^{n + m} a_i t_i \right)\] \[S_1 = \frac{1}{2} \left(\smashoperator[r]{\sum_{i = n + 1}^{n + m}} a_i t_i \right)\] \end{document}