Odtnhj

I've seen that usually, the d'Alembertian is written using the command Box, however, this displays a square with all sides identical. I would like to write it in this other way:

in which, the right and below sides are bolder, so that resembles more to the Laplacian symbol.

To increase the boarder on two sides, you could use the ding113 symbol from pifont

documentclassarticle

usepackagepifont
usepackageamsmath

begindocument

$textding113 Delta$

enddocument

Build a square with thicker lines the same size of square:

documentclassarticle
usepackageamsmath,amssymb

makeatletter
newcommanddalembertianmathopmathpalettedalembertian@relax
newcommanddalembertian@[2]%
 begingroup
 sboxz@$m@th#1square$%
 dimen0=fontdimen8
 ifx#1displaystyletextfontelse
 ifx#1textstyletextfontelse
 ifx#1scriptstylescriptfontelse
 scriptscriptfontfififi3
 makebox[wdz@]%
 hbox to htz@%
 vrule width dimen0
 kern-dimen0
 vbox to htz@
 hrule height dimen0 width htz@
 vss
 hrule height 2dimen0
 %
 kern-2.5dimen0
 vrule width 2.5dimen0
 %
 %
 endgroup

makeatother

begindocument

$dalembertian = mathopDelta-partial_0^2$

$scriptstyledalembertian = mathopDelta-partial_0^2$

$scriptscriptstyledalembertian = mathopDelta-partial_0^2$

Large$dalembertian = mathopDelta-partial_0^2$

enddocument

Just uses rules. Then, scalerel to make it match the height of Delta.

documentclassarticle
usepackagestackengine,scalerel
newcommanddAlaux%
 Shortstackrule12pt.6pt\
 rule.6pt10ptkern10ptrule1.4pt10pt\
 rule12pt1.4pt%

newcommanddAl%
 setstackgapS0pt%
 setstackEOL\%
 scalerel*kern1ptdAlauxkern1ptDelta%

begindocument
$dAl = Delta - partial_0^2,quad phidAlphi e^phidAlphi$
enddocument

A slightly thicker version, by adjusting the rule dimensions...

documentclassarticle
usepackagestackengine,scalerel
newcommanddAlaux%
 Shortstackrule11pt.7pt\
 rule.7pt9ptkern9ptrule1.3pt9pt\
 rule11pt1.35pt%

newcommanddAl%
 setstackgapS0pt%
 setstackEOL\%
 scalerel*kern1ptdAlauxkern1ptDelta%

begindocument
$dAl = Delta - partial_0^2,quad phidAlphi e^phidAlphi$
enddocument

Here's a version using scalebox and overlapping three copies of square so that the thickness can be tweaked. The thickness can be changed by increasing or reducing the second argument to add@dalemb.

For instance, with 03 and 06:

and with 05 and 10:

Thicker than this, another add@dalemb would be needed to avoid white spaces between the squares.

documentclass[a4paper,12pt]article

usepackageamssymb
usepackagegraphicx

makeatletter
DeclareRobustCommanddalembmathpaletteinner@dalemb
definner@dalemb#1%
 add@dalemb#103%
 add@dalemb#106%
 square

defadd@dalemb#1#2%
 sbox0scalebox1.#2$#1square$%
 rlaplower0.#2ht0box0%

makeatother

begindocument

[ Deltadalemb ]
[ partial^Deltadalemb ]
[ partial^partial^Deltadalemb ]

enddocument