The trace of an square matrix is defined to be
(1)
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i.e., the sum of the diagonal elements. The matrix trace is implemented in the Wolfram Language as Tr[list]. In group theory, traces are known as "group characters."
For square matrices and , it is true that
(2)
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(3)
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(4)
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(Lang 1987, p. 40), where denotes the transpose. The trace is also invariant under a similarity transformation
(5)
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(Lang 1987, p. 64). Since
(6)
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(where Einstein summation is used here to sum over repeated indices), it follows that
(7)
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(8)
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(9)
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(10)
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(11)
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where is the Kronecker delta.
The trace of a product of two square matrices is independent of the order of the multiplication since
(12)
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(13)
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(14)
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(15)
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(16)
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(again using Einstein summation). Therefore, the trace of the commutator of and is given by
(17)
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The trace of a product of three or more square matrices, on the other hand, is invariant only under cyclic permutations of the order of multiplication of the matrices, by a similar argument.
The product of a symmetric and an antisymmetric matrix has zero trace,
(18)
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The value of the trace for a nonsingular matrix can be found using the fact that the matrix can always be transformed to a coordinate system where the z-axis lies along the axis of rotation. In the new coordinate system (which is assumed to also have been appropriately rescaled), the matrix is
(19)
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so the trace is
(20)
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where is interpreted as Einstein summation notation.