For a matrix A, the Diagonal(A) command returns the main diagonal of A as a (column) vector.

For a matrix A, the Diagonal(A, k) command returns the (column) vector corresponding to the diagonal of A, specified by the selection parameter k.

For a vector A, the Diagonal(A) command returns a matrix with the entries of A along the main diagonal, and 0 everywhere else.

For a vector A, the Diagonal(A, k) command returns a matrix with the entries of A along the diagonal specified by the selection parameter k, and 0 everywhere else.

The diagonals of a matrix are indexed using signed integers, where the main diagonal has index 0. Superdiagonals are indexed with positive integers and subdiagonals are indexed with negative integers.

This function is part of the ArrayTools package, so it can be used in the short form Diagonal(..) only after executing the command with(ArrayTools). However, it can always be accessed through the long form of the command by using ArrayTools[Diagonal](..).