Dyadic
A linear Polynomial of Dyads 
consisting of nine components 
whichtransform as
 |  |  | (1) |
|  | (2) | |
|  | (3) |
Dyadics are often represented by Gothic capital letters. The use of dyadics is nearly archaic since Tensors perform the same function but are notationally simpler.
A unit dyadic is also called the Idemfactor and is defined such that
 | (4) |
 | (5) |
 | (6) |
References
Arfken, G. ``Dyadics.'' §3.5 in Mathematical Methods for Physicists, 3rd ed. Orlando, FL: Academic Press, pp. 137-140, 1985. Morse, P. M. and Feshbach, H. ``Dyadics and Other Vector Operators.'' §1.6 in Methods of Theoretical Physics, Part I. New York: McGraw-Hill, pp. 54-92, 1953.
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