Voigt notation
Voigt notation is useful to understand the 4th-rank tensors of geophysics, for example the elastic stiffness tensor $ \{C_{ijkl}\} $, needed for wave propagation, or the elastic compliance tensor $ \{S_{ijkl}\} $, needed for geomechanics. These tensors are hard to understand intuitively, since one cannot write them down on paper. (If you use $ i $ to index the rows, and $ j $ to index the columns, where do you put the indices $ k $ and $ l $ ?) But, Woldemar Voigt (1850-1919) realized that because stress and strain are symmetric tensors, and because the order of the index pairs $ ij $ and $ kl $ can be interchanged, all the information in these 4th-rank tensors is contained in symmetric 2nd-rank matrices (which can be written on paper!). (This transform, called Voigt notation, should be used for displays (to aid intuition), not for calculation, since the matrices do not obey tensor algebra.)
For example, the stiffness tensor Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): \{C_{ijkl}\} can be represented as the stiffness matrix Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): \{C_{\alpha\beta}\} :
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): \{C_{ijkl}\} => Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): \{C_{\alpha\beta}\}= \begin{pmatrix} C_{11}&C_{12}&C_{13}&C_{14}&C_{15}&C_{16}\\ C_{12}&C_{22}&C_{23}&C_{24}&C_{25}&C_{26}\\ C_{13}&C_{23}&C_{33}&C_{34}&C_{35}&C_{36}\\ C_{14}&C_{24}&C_{34}&C_{44}&C_{45}&C_{46}\\ C_{15}&C_{25}&C_{35}&C_{45}&C_{55}&C_{56}\\ C_{16}&C_{26}&C_{36}&C_{46}&C_{56}&c_{66}\\ \end{pmatrix}
A common convention is to use Latin indices Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): i (1...3) for the tensors, and Greek indices Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): \alpha (1...6) for the matrices. The mapping of the tensor indices Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): ij to the matrix indices Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): \alpha is:
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): 11 => $ 1 $
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): 22 => Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): 2
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): 33 => Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): 3
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): 23 = Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): 32 => Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): 4
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): 13 = Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): 31 => $ 5 $
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): 12 = Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): 21 => Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): 6
Hence, the compliance matrix above can be written as:
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): \{C_{\alpha\beta}\}= \begin{pmatrix} C_{1111}&C_{1122}&C_{1133}&C_{1123}&C_{1113}&C_{1112}\\ C_{1122}&C_{2222}&C_{2233}&C_{2223}&C_{2213}&C_{2212}\\ C_{1133}&C_{2233}&C_{3333}&C_{3323}&C_{3313}&C_{3312}\\ C_{1123}&C_{2223}&C_{3323}&C_{2323}&C_{2313}&C_{2312}\\ C_{1113}&C_{2213}&C_{3313}&C_{2313}&C_{1313}&C_{1312}\\ C_{1112}&C_{2212}&C_{3312}&C_{2312}&C_{1312}&c_{1212}\\ \end{pmatrix}
(The compliances use a modification of this Voigt notation.)