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I want Mathematica to invert a matrix in which all variables are symbolic including Subscript. Now I am guessing this has something to do with the method I'm defining the variables in the summation, especially the use of Subscript.

But I am worried that since Subscript are for presentation purposes, I don't want to use them and I looking for an alternate way to do this.

As an example, consider the matrix below 

A = { {F11 * Sum[Subscript[towpx^2, i], {i, 1, n}] + 
 F22 * Sum[Subscript[fpx^2, i], {i, 1, n}], 
Sum[F11 * Subscript[towpx, i], {i, 1, n}]*
  Sum[Subscript[towpy, i], {i, 1, n}] + 
 F22 * Sum[Subscript[fpx, i], {i, 1, n}]*
  Sum[Subscript[fpy, i], {i, 1, n}] } , { 
F11* Sum[Subscript[towpx, i], {i, 1, n}]*
  Sum[Subscript[towpy, i], {i, 1, n}] + 
 F22 * Sum[Subscript[fpx, i], {i, 1, n}]*
  Sum[Subscript[fpy, i], {i, 1, n}]  , 
F11 * Sum[Subscript[towpy^2, i], {i, 1, n}] + 
 F22 * Sum[Subscript[fpy^2, i], {i, 1, n}]} };

How can I Invert a matrix that needs to have variables with Subscript for display?

I am very new to Mathematicaand don't understand any of the stuff in general posts regarding correct syntax usages (Apologies if the post isn't appropriate)

Thanks in advance!

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    $\begingroup$ "simplest possible way on how to deal with this issue" - avoid subscripts for computational work, and reserve them for formatting; they're more trouble than they're worth. $\endgroup$
    – J. M.'s missing motivation ♦
    Commented Sep 27, 2017 at 11:13
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    $\begingroup$ Inverse[A] works for me. $\endgroup$
    – Michael E2
    Commented Sep 27, 2017 at 11:13
  • $\begingroup$ @J.M. okay, thank you. but what alternative would you suggest? $\endgroup$
    – S. Khan
    Commented Sep 27, 2017 at 11:16
  • $\begingroup$ @MichaelE2 , yes it does. But I am worried that since subscripts are for presentation purposes, I don't want to use them and I looking for an alternate way to do this. $\endgroup$
    – S. Khan
    Commented Sep 27, 2017 at 11:19
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    $\begingroup$ (1) You might want to edit the question to clarify what you want, since I read the main issue as "I want Mathematica to invert a matrix." (2) The same problem exists for mathematical papers in general. One idea is to represent each type of sum by its own short symbol/variable/string. Or just $A^{-1}$, since formulas for inverses are often unreadably complicated. $\endgroup$
    – Michael E2
    Commented Sep 27, 2017 at 11:33

1 Answer 1

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Rewrite your matrix (notice I'm fixing a the erroneous Subscript[towpx^2, i] that should be Subscript[towpx, i]^2.

b = A /. {Subscript[Power[var_, exp_], index_] -> 
    Power[var[index], exp], Subscript[var_, index_] -> var[index]}

Define an UpValue rule for displaying var[index] as Subscript[var,index] for the vars of interest.

(# /: Format[#[i_]] := Subscript[#, i]) & /@ {fpx, fpy, towpy, towpx}

now

Mathematica graphics

but the FullForm, i.e the form Mathematica uses for internal calculations, is still free of Subscript

Mathematica graphics

and the inverse is just

Inverse[b]
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