山东科技大学2018数学分析712
1、证明:令$u=x^2-y^2,v=2xy.$
$\because \frac{\partial z}{\partial x}=\frac{\partial f}{\partial u}\frac{\partial u}{\partial x}+\frac{\partial f}{\partial v}\frac{\partial v}{\partial x}=2xf_u+2yf_v,$
$\frac{\partial^2 z}{\partial x^2}=\frac{\partial}{\partial x}(\frac{\partial z}{\partial x})=2f_u+4x^2f_{uu}+8xyf_{uv}+4y^2f_{vv}.$
$\frac{\partial z}{\partial y}=\frac{\partial f}{\partial u}\frac{\partial u}{\partial y}+\frac{\partial f}{\partial v}\frac{\partial v}{\partial y}=-2yf_u+2xf_v.$
$\frac{\partial^2 z}{\partial y^2}=\frac{\partial}{\partial y}(\frac{\partial z}{\partial y})=-2f_{u}-4x^2f_{uu}-8xyf_{uv}-4y^2f_{vv}.$
$\therefore \frac{\partial^2 z}{\partial x^2}+\frac{\partial^2 z}{\partial y^2}=0.$
2、证明:
$\because I(\alpha )=\int_{0}^{+\infty }\sqrt{\alpha }e^{-\alpha x^2}dx=\int_{0}^{+\infty }e^{-(\sqrt{\alpha} x)^2}d(\sqrt{\alpha }x)=\frac{\sqrt{\pi}}{2} .(\alpha \not\equiv 0)$
所以,收敛。
又因为
$I(\alpha )=\begin{cases}
\frac{\sqrt{\pi}}{2}, &\alpha\neq 0 \\
0,&\alpha =0
\end{cases}$
显然,参变量积分在$\alpha =0$不连续,因此积分不一致收敛。