依题意,a,b 应该是同维列向量
a=(a1,...,an)^T,b=(b1,...,bn)^T
ab^T=
a1b1 a1b2 ...a1bn
a2b1 a2b2 ...a2bn
...
anb1 anb2 ...anbn
所以 ab ^T的对角元之和 tr(ab^T) = a1b1+a2b2+...+anbn = a^T
依题意,a,b 应该是同维列向量
a=(a1,...,an)^T,b=(b1,...,bn)^T
ab^T=
a1b1 a1b2 ...a1bn
a2b1 a2b2 ...a2bn
...
anb1 anb2 ...anbn
所以 ab ^T的对角元之和 tr(ab^T) = a1b1+a2b2+...+anbn = a^T