30. Geometry of determinants, 2

30. Geometry of determinants, 2

Higher dimensions

Last time we saw a geometric interpretation of the determinant of a 2-by-2 matrix A as the area of a parallelogram obtained by applying A to a square. This result holds in all dimensions, though we will not prove it:

Theorem:

If A is an n -by-n matrix and S𝐑n is the unit n -dimensional cube then |det(A)|=vol(A(S)) .

Remark:

The absolute value signs around the determinant need to be there because areas and volumes are positive but determinants can be negative. In the proof of the 2-by-2 case, I hid the issue in the picture I drew: if I had switched the vectors (ac) and (bd) in the picture, we would have ended up with bc-ad , i.e. -det(A) .

Remark:

In higher dimensions, A(S) is no longer a parallelogram, but a parallelopiped ("parallel-legged"): a shape whose faces are parallel pairs of congruent parallelograms.

Tetrahedra/simplices

Another useful characterisation of the determinant (which we won't prove) is the following.

Theorem:

Let a1,,an be vectors in 𝐑n . Consider the simplex with vertices at 0,a1,,an . Then the volume of this simplex is 1n!det(A) where A is the matrix with columns a1,,an .

Remark:

A simplex is the simplest convex shape you can draw with these vertices: you take the vertices, connect them by lines, and then fill in all the space in between. (More formally, this is called taking the convex hull of the vertices). In 2 dimensions this gives a triangle (3 vertices). In 3 dimensions you get a tetrahedron.

A triangle and a tetrahedron (2- and 3-dimensional simplices)

These two theorems are closely related: you can decompose a cube into n! simplices. Rather than proving either of them, I want to use this second theorem to calculate the volumes of some tetrahedra in 𝐑3 .

Example:

Take a1=(100) , a2=(010) and a3=(001) . The corresponding tetrahedron has volume 16det(100010001)=16 .

The tetrahedron with vertices at (0,0,0), (1,0,0), (0,1,0) and (0,0,1)
Example:

Take a regular tetrahedron (d4 for the roleplayers out there) with vertices at a0=(111) , a1=(1-1-1) , a2=(-11-1) and a3=(-1-11) . (These vertices are half of the vertices of a regular cube centred at the origin with sidelength 2.)

A regular tetrahedron (4-sided die!)

Translating by -a0 , we get the vertices 0,a1-a0,a2-a0,a3-a0 , so we can apply our theorem with A=(a1-a0a2-a0a3-a0)=(0-2-2-20-2-2-20).

We compute the determinant using the cofactor expansion along the top row: vol=|16det(0-2-2-20-2-2-20)|
=16|(-(-2)det(-2-2-20)-2det(-20-2-2)|
=26|(-4-4)|
=166

Determinant as scale factor for volumes

det(A) is the scale factor for volumes of cubes under the transformation A . Since any volume is defined by subdividing/approximating by smaller and smaller cubes, det(A) is the scale factor for any volume under the transformation A . From this point of view, the formula det(AB)=det(A)det(B) is just telling us that if we apply the transformation B and then the transformation A , the volumes rescale by det(B) and then by det(A) , giving a total scale factor of det(A)det(B) for the composite transformation.