What can we say about U? Well, if we fix a v3∈V3, then it will appear in (N1n1)(N2n2)(N3−1n3−1) of the Uσ terms (i.e. all choices of the V1 and V3 variables, and all possible choices of the other n3−1 variables in the other N3−1 variables in V3).
as in
(i.e. all choices of the V1 and V2 variables, and all possible choices of the other n3−1 variables in the other
These might be some typos:
From the following, the second V3 should be V2:
as in
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