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# Skew-symmetric matrix

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In linear algebra, a square matrix A is said to be skew-symmetric if its transpose is also its negative; that is, it satisfies the equation:

AT = -A

or in component form, if A = (ai,j):

ai,j = - aj,i   for all i and j

For example, the following matrix is skew-symmetric:

$\begin{bmatrix} 0 & 2 \\ -2 & 0 \end{bmatrix}$

All main diagonal entries of a skew-symmetric matrix have to be zero, and so the trace is zero.

The skew-symmetric n-by-n matrices form a vector space of dimension (n2 - n)/2. This is the tangent space to the orthogonal group O(n). In a sense, then, skew-symmetric matrices can be thought of as "infinitesimal rotations".

In fact, the skew-symmetric n-by-n matrices form a Lie algebra using the commutator Lie bracket

$[A,B] = AB - BA\,$
and this is the Lie algebra associated to the Lie group O(n).

A matrix G is orthogonal and has determinant 1, i.e., it is a member of that connected component of the orthogonal group in which the identity element lies, precisely if for some skew-symmetric matrix A we have

$G=\exp(A)=\sum_{n=0}^\infty \frac{A^n}{n!}.$

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