  Let $A \in {\cal C}^n$ be an extremal copositive matrix with unit diagonal.
Then the minimal zeros of $A$ all have supports of cardinality two if and only
if the elements of $A$ are all from the set $\{-1,0,1\}$. Thus the extremal
copositive matrices with minimal zero supports of cardinality two are exactly
those matrices which can be obtained by diagonal scaling from the extremal
$\{-1,0,1\}$ unit diagonal matrices characterized by Hoffman and Pereira in
1973.
