# Prove that det(A^2) = 1

• Mar 29th 2011, 05:53 PM
Lord Darkin
Prove that det(A^2) = 1
If A is row equivalent to I and I can be obtained from A by using only row operations of type I (interchange rows) and III (adding a multiple of one row to another), prove $det(A^2) = 1$.

My attempt:

If A is row equivalent to I, then A is the product of some elementary matrices. As a result, det(EA) = det(E)det(A) where E is some elementary matrix.

*Assume det(A) is multiplied by the determinants of elementary matrices of type I. When this happens, that will change the value of det(A) by -1 since det(E) = -1 and det(EA) = det(E)det(A)

*Similarly, assume det(A) is multiplied by the determinants of elementary matrices of type III. When this happens, det(A) is unchanged because det(E) = 1 and det(EA)=det(E)det(A)

We can also consider the case with both row operations I and III, which again, will only change the det(A) by multiplying it by -1 or leaving it unchanged.

A is row equivalent to I, so

A = (E1)(E2) *** (Ek)(I)

This implies that

det(A) = det[(E1)(E2) *** (Ek)(I)]
det(A) = det(E1)det[(E2) *** (Ek)(I)]
...
det(A) = det(E1)det(E2) *** det(Ek)det(I)

The determinant of the identity matrix I is 1 since the identity matrix is triangular, and the product of its diagonal elements is 1. Therefore, multiplying it by the determinants of elementary matrices I and III mean only two possible values of det(A) exist: 1 and -1. Squaring that in det(A^2) mean that det(A^2) = 1.

Proof done.

Note: The proofs for why det(E) = 1 or -1 where E is type I or III, respectively, are already in my textbook, so I can simply cite the proofs there. I inserted an asterik (*) at the start of those 2 statements just to make things clear for you as viewers.
• Mar 30th 2011, 12:31 AM
ojones
This argument is correct; well done! At the end you can say $\text{det}(A^2)=[\text{det}(A)]^2$ and so the minus sign doesn't matter.
• Mar 30th 2011, 06:38 PM
Lord Darkin
Thanks for the comment, I'll include that at the end of my proof.