hmm.. that seems pretty clear to me : here

The problem arises in something we're looking at involving quantum ent
+anglement - each of the a[i][0] + a[i][1] terms (i=0, 1, 2, ..., N-1)
+ represents a two-level quantum state. N such terms multiplied togeth
+er thus represents a product state of N two-level states. We're then 
+seeing if a general state can be written in this form - if it cannot,
+ then the degree to which it can't is a measure of the degree of enta
+nglement.
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