# diagonals of tensors

**Thomas Lumley
**
tlumley@u.washington.edu

*Fri, 26 Jul 2002 11:58:47 -0700 (PDT)*

Is there a simple and efficient way of extract `diagonals' from arrays
(tensors)
That is, suppose A is a 20x10x2 array, B is 10x5 and C is 20x5 I would
like the 2x5 matrix whose i,j entry is
sum_k sum_l A(k,l,i)B(l,j)C(k,j)
Using tensor() (or outer and matrix multiplication) it's easy to construct
the 2x5x5 matrix with i,j,J entry
sum_k sum_l A(k,l,i)B(l,j)C(k,J)
[eg tensor(tensor(A,B,2,1),C,1,1) ]
and I'm looking for the j==J `diagonal', ideally without constructing the
whole thing.
-thomas
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