这么不规范，真郁闷
这个式子里Y的脚标都有j，所以认为是“K与Y的乘积都求和”

但是，不应该有个括号把K,Y括起来吗？

楼主的问题我回答不了，但是你怎么重复回帖那么多次啊。等你解决了问题告诉我一下，我把这个帖子删掉

Yes, it should be the sum of the products between K and Y, since the summation is taken over j. You could put brackets around K and Y, but it not omitted here because it does not cause confusion if you properly understand the definition of summation notation.