性质1 (线性变换 保零 保负)

$\sigma \left(0\right)=0,\sigma \left(-\alpha \right)=-\sigma \left(\alpha \right)$.

实际上,

$$\sigma \left(0\right)=\sigma \left(0\alpha \right)=0\sigma \left(\alpha \right)=0,$$ $$\sigma \left(-\alpha \right)=\sigma \left(\left(-1\right)\alpha \right)=\left(-1\right)\sigma \left(\alpha \right)=-\sigma \left(\alpha \right).$$