Mode-k flattening

In multi-linear algebra, mode-k flattening (also matricisation, matricizing, or unfolding) is an operation on tensor (a multi-dimensional array) formula_1 denoted by formula_2 turning it into a matrix (a two-dimensional array).
Matricization may be regarded as a generalization of the mathematical concept of vectorization.
Matricization may be applied in connection with determination of the factors in the PARAFAC model.
Applications.
This operation is used widely in HoSVD.
Also, there is a mode-k flattening representation of SVD.