A tensor is said to satisfy the double contraction relation when

(1)

This equation is satisfied by
where the hat denotes zero trace, symmetric unit tensors. These tensors are used to define the spherical
harmonic tensor.
See also
Spherical Harmonic Tensor,
Tensor
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References
Arfken, G. "Alternating Series." Mathematical Methods for Physicists, 3rd ed. Orlando, FL: Academic Press, p. 140,
1985.Referenced on WolframAlpha
Double Contraction Relation
Cite this as:
Weisstein, Eric W. "Double Contraction Relation."
From MathWorldA Wolfram Web Resource. https://mathworld.wolfram.com/DoubleContractionRelation.html
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