9. [HM46] Let us consider condition (5.6.110). The proof that the error function is local minima free given in Section 5.6.1 assumes that X1 is a full rank matrix. While this is generally true whenever condition (5.6.110) is met, there are configurations with rank deficiency, that is, weights w such that rankX1(w) < min{, 1 + |H |}. Prove that also these configurations are not local minima. This problem has been addressed in [278], [349], [350], [351]. Some comments published in [164] have raised doubts on the proofs.