设上三角形的正交矩阵A=[a1,a2,...,an]
a1=(a11,0,...,0)^T,a2=(a12,a22,0,...,0)^T,...,an=(a1n,a2n,...,ann)(akk≠0,k=1,2,...,n)
由a1^T*ak=0(k≠1)得：a11*a1k=0,即a1k=0(k=2,3,...,n)
同理：aij=0(i