$$
\varphi_x(t)=\int_{-\infty}^{\infty} e^{i t x} f(x) \cdot d x
$$
独立变量和的特征函数
$Y=X_1+X_2 $,其中$X_1,X_2$相互独立,特征函数:
$$
\begin{array}{l}
\varphi_{y}(t)&=\varphi_{x_{1}+x_{2}}(t) \\
&=\iint_{-\infty}^{\infty} e^{i t\left(x_{1}+x_{2}\right)} \cdot f\left(x_{1}\right) \cdot g\left(x_{2}\right) \cdot d x_{1} d x_{2} \\
&=\int_{-\infty}^{\infty} e^{i t x_{1}} f\left(x_{1}\right) d x_{1} \cdot \int_{-\infty}^{\infty} e^{i t x_{2}} g\left(x_{2}\right) d x_{2} \\
&=\varphi_{x_{1}}(t) \cdot \varphi_{x_{2}}(t)
\end{array}
$$
常数线性变换的特征函数
$Y=aX+b$ 的特征函数:
$$
\begin{array}{l}
\varphi_{y}(t)&=\varphi_{a x+b}(t)=\int_{-\infty}^{\infty} e^{i t(a x+b)} f(x) d x \\
&=e^{i t b} \int_{-\infty}^{\infty} e^{i(a t) x} f(x) d x \\
&=e^{i t b} \cdot \varphi_{x}(a t)
\end{array}
$$