Let A = bmatrix 0 & 2 & -3 \ -2 & 0 & 1 \ 3 & -1 & 0 bmatrix and B be a matrix such that B(I - A) = I + A. Then the sum of the diagonal elements of BTB is equal to
Answer
3
Solution
AT = -A, i.e. A is skew-symmetric.
|I - A| = 1 + (22 + 32 + 12) = 15 0 I - A is invertible
B = (I + A)(I - A)-1
BT = [(I - A)-1 ]T(I + A)T = (I + A)-1(I - A) ( AT = -A)
BTB = (I + A)-1(I - A)(I + A)(I - A)-1
(I - A)(I + A) = I - A2 = (I + A)(I - A)
BTB = (I + A)-1(I + A)(I - A)(I - A)-1 = I
Sum of diagonal elements of BTB = tr(I3) = 3
Answer: 3