Diagonal

WMA link

Diagonal[m]

gives a list with the values in the diagonal of the matrix m.

Diagonal[m, k]

gives a list with the values in the k diagonal of the matrix m.

• • Diagonal[{{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}]

Get the superdiagonal:

• • Diagonal[{{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}, 1]

Get the subdiagonal:

• • Diagonal[{{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}, -1]

Get the diagonal of a nonsquare matrix:

• • Diagonal[{{1, 2, 3}, {4, 5, 6}}]