【对偶空间】
dual vector space
对偶空间是线性函数的集合。
Dual vector space consists of linnear functions.
In mathematics, any vector space V has a corresponding dual vector space (or just dual space for short) consisting of all linear functionals on V,
together with the vector space structure of pointwise addition and scalar multiplication by constants.
原文：http://www.cnblogs.com/yuanjiangw/p/7531333.html