Up arrow under a sequence

Question

. It is a mathematical expression/sequence/set of numbers with the arrow exactly under any one of the numbers (with or without n= 0). How to do it in latex? Any of the up arrow types (single line or double line) shown in the figure is okay.

campa · Accepted Answer

Basically the same as Werner's answer but with some wrapping around it:

documentclass{article}

usepackage{amsmath}

makeatletter

newcommand*{underarrow}{def@underarrow{relax}@ifstar{@@underarrow}{def@underarrow{hidewidth}@@underarrow}}
newcommand*{@@underarrow}[2][]{underset{@underarrowsubstack{uparrowifrelaxdetokenize{#1}relaxelse#1fi}@underarrow}{#2}}

newcommand*{overarrow}{def@overarrow{relax}@ifstar{@@overarrow}{def@overarrow{hidewidth}@@overarrow}}
newcommand*{@@overarrow}[2][]{overset{@overarrowsubstack{ifrelaxdetokenize{#1}relaxelse#1fidownarrow}@overarrow}{#2}}
makeatother

begin{document}

[
(1, 2, 3)
= (underarrow{1}, 2, overarrow{3})
= (1, underarrow{2}, 3)
= (overarrow[n=1]{1}, 2, underarrow{3})
= (1, underarrow[n = 2]{2}, 3)
= (underarrow*[n=1]{1}, underarrow*[n = 2  n=2]{2}, 3)
]

end{document}

The extra text can be given as optional parameter. The unstarred version ignores the width of the optional parameter, the starred doesn't.

Werner · Answer

A simple uparrow can be placed using underset{uparrow}{<stuff>}. If you're adding wide content, you may want to consider stacking them (using substack - a small array structure) and using mathclap (from mathtools).

documentclass{article}

usepackage{mathtools}

begin{document}

[
  x_1(n) = (1, 2, 3)
         = (underset{uparrow}{1}, 2, 3)
         = (1, underset{uparrow}{2}, 3)
         = (1, 2, underset{uparrow}{3})
         = (1, underset{mathclap{substack{uparrow  n = 2}}}{2}, 3)
]

end{document}

Cicada · Answer

A lower-level method, adapting an answer by @wipet at Arrow under text, uses mathbin with the formatting provided by limits..., plus boxes and kerning:

MWE
documentclass{article}

%%From:
%https://tex.stackexchange.com/questions/554923/arrow-under-text
%deftype#1#2{mathbin{mathop{#1kern0pt}%
%   limits_{raise2.5exhbox to0pt{hssstrut$uparrow$hss}hbox to0pt{hss#2hss}}}}

newcommandtypel[2]{%
mathbin{mathop{#1kern0pt}%
   limits_{raisebox{2.5ex}{hbox to0pt{hssstrut$uparrow$hss}}hbox to0pt{hss#2hss}}}
}

newcommandtypem[2]{%
mathbin{mathop{#1kern0pt}%
   limits^{raisebox{2.5ex}{hbox to0pt{hss#2hss}}hbox to0pt{hssstrut$downarrow$hss}}}
}

begin{document}

[
  x_1(n) = (1, 2, 3)
         = (typel{1}{$n=1$}, 2, 3)
         = (1, typel{2}{even}, 3)
         = (1, 2, typel{3}{})
         = (1, typel{2}{$n = 2$}, typem{3}{odd})
]

end{document}

Up arrow under a sequence

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