fix http://disq.us/p/25jjv9w

docs/09-Additive-Models-Trees-and-Related-Methods/9.2-Tree-Based-Methods.md

@@ -47,7 +47,7 @@ R_1(j,s)=\{X\mid X_j\le s\}\;\text{and}\; R_2(j,s)=\{X\mid X_j>s\}\tag{9.12}
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 于是我们寻找分离变量$j$和分离点$s$求解
 $$
-\underset{j,s}{\mathrm {min}}[\underset{c_1}{\mathrm{min}}\;\sum\limits_{x_i\in R_1(j,s)}(y_i-c_1)^2+\underset{c_1}{\min}\sum\limits_{x_i\in R_2(j,s)}(y_i-c_2)^2]\tag{9.13}
+\underset{j,s}{\mathrm {min}}[\underset{c_1}{\mathrm{min}}\;\sum\limits_{x_i\in R_1(j,s)}(y_i-c_1)^2+\underset{c_2}{\min}\sum\limits_{x_i\in R_2(j,s)}(y_i-c_2)^2]\tag{9.13}
 $$
 对于每个选定的$j$和$s$，内部最小化通过下式来求解
 $$