If A=(abcd)A = \begin{pmatrix} a & b \\ c & d \end{pmatrix}A=(acbd), what is ATA^TAT?
(acbd)\begin{pmatrix} a & c \\ b & d \end{pmatrix}(abcd)
(badc)\begin{pmatrix} b & a \\ d & c \end{pmatrix}(bdac)
(cdab)\begin{pmatrix} c & d \\ a & b \end{pmatrix}(cadb)
(abcd)\begin{pmatrix} a & b \\ c & d \end{pmatrix}(acbd)