1  0  0
0  1  0
0  0  1

Transpose of I

For any dimension:

I^T = I

Another useful property is the following. Actually, this is really just an application of the definition of I.

A I B = A B

Another easy application of the definition:

I I = I, or I² = I

These are all obvious; you should not have to memorize anything here. But it does help to look them over.

Compute the following:

0.5  0.0  0.0
0.0  0.5  0.0
0.0  0.0  0.5

2    4    6
0  -12   -8
4    0    4

Hint: you don't actually need to multiply matrices.