Which matrices are symmetric (i.e., A=ATA = A^TA=AT)?
(1234)\begin{pmatrix} 1 & 2 \\ 3 & 4 \end{pmatrix}(1324)
(1224)\begin{pmatrix} 1 & 2 \\ 2 & 4 \end{pmatrix}(1224)
(5005)\begin{pmatrix} 5 & 0 \\ 0 & 5 \end{pmatrix}(5005)
(1324)\begin{pmatrix} 1 & 3 \\ 2 & 4 \end{pmatrix}(1234)