  In representation theory, a classification problem is called wild if it
contains the problem of classifying matrix pairs up to simultaneous similarity.
The latter problem is considered as hopeless; it contains the problem of
classifying an arbitrary finite system of vector spaces and linear mappings
between them. We prove that an analogous "universal" problem in the theory of
tensors of order at most 3 over an arbitrary field is the problem of
classifying three-dimensional arrays up to equivalence transformations \[
[a_{ijk}]_{i=1}^{m}\,{}_{j=1}^{n}\,{}_{k=1}^{t}\ \mapsto\ \Bigl[ \sum_{i,j,k}
a_{ijk}u_{ii'} v_{jj'}w_{kk'}\Bigr]{}_{i'=1}^{m}\,{}_{j'=1}^{n}\,{}_{k'=1}^{t}
\] in which $[u_{ii'}]$, $[v_{jj'}]$, $[w_{kk'}]$ are nonsingular matrices:
this problem contains the problem of classifying an arbitrary system of tensors
of order at most three.
