Question
Let $$A, B , C, D$$ be (not necessarily square) real matrices such that $$A^T = BCD; B^T = CDA; C^T = DAB$$ and $$DT = ABC$$ for the matrix $$S = ABCD, S^3 =$$
A.
$$I$$
B.
$$S^2$$
C.
$$S$$
D.
$$O$$
Answer :
$$S$$
Solution :
$$\eqalign{
& S = ABCD = A\left( {BCD} \right) = A{A^T}\,\,\,\,.....\left( 1 \right) \cr
& {S^3} = \left( {ABCD} \right)\left( {ABCD} \right)\left( {ABCD} \right) \cr
& = \left( {ABC} \right)\left( {DAB} \right)\left( {CDA} \right)\left( {BCD} \right) \cr
& = {D^T}{C^T}{B^T}{A^T} \cr
& = {\left( {BCD} \right)^T}{A^T} = A{A^T}\,\,\,\,\,\,\,.....\left( 2 \right) \cr} $$
From (1) and (2), $$S = {S^3}$$