# 根据 a 求 w 和 b\*

a\* 为对偶问题的最优解，则：

$$
w\* = \sum\_{i=1}^Na^\*\_iy\_ix\_i
$$

选择a\*中一个满足$$0 \lt a\_j \lt C$$的分量$$a\_j$$，则

$$
b\* = y\_j - \sum\_{i=1}^Ny\_ia^\*\_i(x\_i \cdot x\_j)
$$

任意一个符号条件的$$a\_j$$$都能求出一个b*，*\
*最终的 b* 为所有满足条件的 b\* 的平均值。


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