+ sage: A = zero_matrix(QQ, 5, 5)
+ sage: has_admissible_extreme_rank(A)
+ True
+
+ Likewise, rank one is admissible for dimension 5::
+
+ sage: v = vector(QQ, [1,2,3,4,5])
+ sage: A = v.column()*v.row()
+ sage: has_admissible_extreme_rank(A)
+ True
+
+ But rank 2 is never admissible::
+
+ sage: v1 = vector(QQ, [1,0,0,0,0])
+ sage: v2 = vector(QQ, [0,1,0,0,0])
+ sage: A = v1.column()*v1.row() + v2.column()*v2.row()
+ sage: has_admissible_extreme_rank(A)
+ False
+
+ In dimension 5, three is the only other admissible rank::
+
+ sage: v1 = vector(QQ, [1,0,0,0,0])
+ sage: v2 = vector(QQ, [0,1,0,0,0])
+ sage: v3 = vector(QQ, [0,0,1,0,0])
+ sage: A = v1.column()*v1.row()
+ sage: A += v2.column()*v2.row()
+ sage: A += v3.column()*v3.row()
+ sage: has_admissible_extreme_rank(A)
+ True