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The elements on the diagonal of a skew-symmetric matrix are zero, and therefore its trace equals zero. If is a real skew-symmetric matrix and is a real eigenvalue, then =, i.e. the nonzero eigenvalues of a skew-symmetric matrix are non-real. If is a real skew-symmetric matrix, then + is invertible, where is the identity matrix.
Connecting the Lie algebra to the Lie group is the exponential map, which is defined using the standard matrix exponential series for e A [13] For any skew-symmetric matrix A, exp(A) is always a rotation matrix. [nb 3] An important practical example is the 3 × 3 case.
[nb 2] The Lie bracket of two elements of () is, as for the Lie algebra of every matrix group, given by the matrix commutator, [A 1, A 2] = A 1 A 2 − A 2 A 1, which is again a skew-symmetric matrix. The Lie algebra bracket captures the essence of the Lie group product in a sense made precise by the Baker–Campbell–Hausdorff formula.
Let A = (a ij) be a 2n × 2n skew-symmetric matrix. The Pfaffian of A is explicitly defined by the formula =! = (), (), where S 2n is the symmetric group of degree 2n and sgn(σ) is the signature of σ.
If instead, A is equal to the negative of its transpose, that is, A = −A T, then A is a skew-symmetric matrix. In complex matrices, symmetry is often replaced by the concept of Hermitian matrices, which satisfies A ∗ = A, where the star or asterisk denotes the conjugate transpose of the matrix, that is, the transpose of the complex ...
Similarly in characteristic different from 2, each diagonal element of a skew-symmetric matrix must be zero, since each is its own negative. In linear algebra, a real symmetric matrix represents a self-adjoint operator [1] represented in an orthonormal basis over a real inner product space.
The geometric interpretation of curl as rotation corresponds to identifying bivectors (2-vectors) in 3 dimensions with the special orthogonal Lie algebra of infinitesimal rotations (in coordinates, skew-symmetric 3 × 3 matrices), while representing rotations by vectors corresponds to identifying 1-vectors (equivalently, 2-vectors) and ...
Conversely, let Q be any orthogonal matrix which does not have −1 as an eigenvalue; then = (+) is a skew-symmetric matrix. (See also: Involution.) The condition on Q automatically excludes matrices with determinant −1, but also excludes certain special orthogonal matrices.