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I want to force the operands of a sum to be placed above an "extended" (longer than usual) index, instead of increasing the width of the sum (in the example, see the sums in (2), which are much wider than in (1), because of the different index). I need this for space reasons, the formula is bigger than in the MWE. Do you have any ideas how to achieve this?

documentclass[12pt]{article}

usepackage{amsmath}



begin{document}



begin{align}

  text{textbf{Total Cost }} 

    &= sum_{m=1}^{55} 10,000 cdot Z_m + 7,000 cdot K_m \

    &= sum_{i in {m | Z_m = 1}} 10,000 + sum_{j in {k | K_m = 1}} 

7,000

end{align}



end{document}

Use mathclap from mathtools package.

documentclass[12pt]{article}

%usepackage{amsmath}

usepackage{mathtools}



begin{document}

begin{align}

  text{textbf{Total Cost }} 

    &= sum_{m=1}^{55} 10,000 cdot Z_m + 7,000 cdot K_m \

    &= sum_{mathclap{i in {m | Z_m = 1}}} 10,000 + sum_{mathclap{j in {k | K_m = 1}} }

7,000

end{align}

end{document}

mathtools loads amsmath.

documentclass[12pt]{article}

usepackage{mathtools}



begin{document}



begin{align}

  textbf{Total Cost} % no trailing space inside braces

    &= sum_{m=1}^{55} 10,000 cdot Z_m + 7,000 cdot K_m \

    &= sum_{mathclap{i in {m, | , Z_m = 1}}} 10,000 + sum_{mathclap{j in {k, | , K_m = 1}}} 7,000 

end{align}



end{document}

A variant, with the smashoperator command from mathtools – and some improvements with siunitx, so the comma separator in numbers doesn't add a space.

smashoperator can take an optional argument, [l] or [r] which are equivalent to mathlap or mathrlap respectively. I demonstrate it in a $3$rd equation:

documentclass[12pt]{article}

usepackage{mathtools}

usepackage{siunitx}



begin{document}



sisetup{group-digits = integer, group-separator={,}, group-minimum-digits = 4}

begin{align}

  textbf{Total Cost} % no trailing space inside braces

    &= sum_{m=1}^{55} num{10000} cdot Z_m + num{7000} cdot K_m \

    &= smashoperator{sum_{i in {mmid Z_m = 1}}}num{10000} + smashoperator{sum_{j in {kmid K_m = 1}}} num{7000} \

    &= smashoperator[r]{sum_{i in {mmid Z_m = 1}}} num{10000} + smashoperator[l]{sum_{j in {kmid K_m = 1}}} num{7000}

end{align}



end{document}