I would creat a symmetric matrix, but how can I set the boundary in Subscript?
Array[Subscript[a, #1, #2] &, {3, 3}] // MatrixForm
$a_{ij} = a_{ji}$, $j$ is not equal to $i$.
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$\begingroup$ may be mathematica.stackexchange.com/questions/7887/… $\endgroup$– NasserCommented Jul 19, 2018 at 0:35
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Array[Subscript[a, ## & @@ Sort[{##}]] &, {5, 5}] // MatrixForm // TeXForm
$\left( \begin{array}{ccccc} a_{1,1} & a_{1,2} & a_{1,3} & a_{1,4} & a_{1,5} \\ a_{1,2} & a_{2,2} & a_{2,3} & a_{2,4} & a_{2,5} \\ a_{1,3} & a_{2,3} & a_{3,3} & a_{3,4} & a_{3,5} \\ a_{1,4} & a_{2,4} & a_{3,4} & a_{4,4} & a_{4,5} \\ a_{1,5} & a_{2,5} & a_{3,5} & a_{4,5} & a_{5,5} \\ \end{array} \right)$
You can also use
Array[Subscript[a, Min @ ##, Max @ ##] &, {5, 5}]
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$\begingroup$ thanks a lot! how can I get the voigt notation from second order tensor in a efficient way in Mathematica? $\endgroup$– ABCDEMMMCommented Jul 19, 2018 at 21:19
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$\begingroup$ @ABCDEMMM, i am afraid i don't know off the top of my head; that sounds like good new question. $\endgroup$– kglrCommented Jul 19, 2018 at 21:25
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$\begingroup$ I am also afraid that I will I get negative note if i ask such a question.... $\endgroup$– ABCDEMMMCommented Jul 19, 2018 at 21:27
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$\begingroup$ @ABCDEMMM, i happen to think that it is a good question. With a small example and a link (say, to wikipedia) it is likely to be received very well on this site. $\endgroup$– kglrCommented Jul 19, 2018 at 21:34