https://baike.baidu.com/item/格拉姆矩阵/16274086
https://en.wikipedia.org/wiki/Gramian_matrix
https://en.wikipedia.org/wiki/Matrix_congruence
https://en.wikipedia.org/wiki/Congruence_relation
格拉姆矩阵 合同矩阵
原文：https://www.cnblogs.com/yuanjiangw/p/10559870.html
