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In the question Geometric interpretations of matrix inverses, I provided a visual answer for the $2 \times 2$ case, showing the relationship between the matrices $A$ and $A^{-1}$ in terms of the size and shapes of the parallelograms of their row vectors.

I've completed a parallel demonstration for the inverse of a $3 \times 3$ matrix, shown below, but I would like to characterize the relationship more completely than I did before. For example,

  • What is a general statement of the orthogonality relations between the (row) vectors $a_1$, $a_2$, and $a_3$ that define the paralellepiped of $A$ to the corresponding vectors $a^1$, $a^2$, and $a^3$ of $A^{-1}$?

  • Are there interesting relations between dot (inner) products and cross-products of the vectors $a_i$ and $a^i$?

  • How can one describe the relationships between the faces of $A$ and $A^{-1}$?

In this example, I use the following matrix A, whose determinant is 2.

     [,1] [,2] [,3]
[1,]    1    0    1
[2,]    0    2    0
[3,]    1    0    2 

Its inverse is:

     [,1] [,2] [,3]
[1,]    2  0.0   -1
[2,]    0  0.5    0
[3,]   -1  0.0    1

Here is a 3D plot of $A$ and $A^{-1}$, with axes and some corresponding vertex points labeled. Faces are colored in the same way on both views.

enter image description here

Here is an animated version of the same, rotating around the $z$ axis.

enter image description here

In case anyone is interested, these figures are done in R, and the code for this example is in this gist

  • 2
    Very nice, both the drawings and the code. – marty cohen Sep 30 '16 at 15:58
  • @user101089 I tried to use the code that you provided but it is giving me the following error:

    Error in movie3d(spin3d(rpm = 15), duration = 4, movie = "inv-demo", dir = ".") : could not find function "movie3d"

    – rose May 08 '19 at 17:19

0 Answers0