二次判别函数的决策面是超二次曲面,包括超平面、超平面对、超球面、超椭球面、超抛物面、超双曲面。
二次判别函数f(x1,x2)f(x_1, x_2)f(x1,x2)
f(x1,x2)=x⊤Wx+w⊤x+b f(x_1, x_2) = {\boldsymbol x}^ { \top} {\boldsymbol{{W}}}{\boldsymbol x} + \boldsymbol{w}^ { \top} \boldsymbol{x} + {b}f(x1,x2)=x⊤Wx+w⊤x+b
W=[w11w12w21w22] {\boldsymbol W} = \begin{bmatrix} w_{11} & w_{12} \\ w_{21} & w_{22} \end{bmatrix}W=[w11w21w12w22]w=[w1w2] {\boldsymbol w} = \begin{bmatrix} w_1 \\ w_2 \end{bmatrix}w=[w1w2]x=[x1x2] {\boldsymbol x} = \begin{bmatrix} x_1 \\ x_2 \end{bmatrix}x=[x1x2]
f(x1,x2)=w11x12+(w12+w21)x1x2+w22x22+w1x1+w2x2+w0 f(x_1, x_2) = w_{11}x_1^2 + (w_{12} + w_{21})x_1x_2 + w_{22}x_2^2+ w_1x_1 + w_2x_2 + w_{0}f(x1,x2)=w11x12+(w12+w21)x1x2+w22x22+w1x1+w2x2+w0
决策面方程
f(x1,x2)=0 f(x_1, x_2) = 0f(x1,x2)=0