正文
設動力體系為
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
利用(8),對已給的 ζ、η,亦即已給的群 (2),可以決定最一般的F(x,y),使方程(5)在群(2)之下不變。當 ζ、η、F一起滿足(8)時,若令
![常微分方程變換群理論](/img/9/ec5/ml2ZuM3X4QTNyYTM3UTNxgDM5ETMwADMwADMwADMwADMxAzL1EzL4QzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
(9)
特別,在平移群x1=x+t,y1=y(此時ζ=1,η=0,由(8)可解出F=ƒ(y))之下為不變的方程(5)取
在均勻放大群x1=kx,y1=ky(令k=et即見ζ=x,η=y)之下為不變的方程(5)是齊次方程
![常微分方程變換群理論](/img/c/eb4/ml2ZuM3XwADO4YTM3UTNxgDM5ETMwADMwADMwADMwADMxAzL1EzLwAzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![常微分方程變換群理論](/img/7/62a/ml2ZuM3X0YDO5YTM3UTNxgDM5ETMwADMwADMwADMwADMxAzL1EzL0YzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
利用這種方法就可看出,許多方程之所以能用初等積分法求解,都是因為使它們不變的變換群(2)是一些易於求解的方程(1)的解。
從理論上講,(1)的通積分可表為
(10)
![常微分方程變換群理論](/img/e/4c7/ml2ZuM3X1gTM0cTM3UTNxgDM5ETMwADMwADMwADMwADMxAzL1EzL1gzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![常微分方程變換群理論](/img/f/c62/ml2ZuM3X2MDM2cTM3UTNxgDM5ETMwADMwADMwADMwADMxAzL1EzL2MzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![常微分方程變換群理論](/img/b/52e/ml2ZuM3X0gTO2cTM3UTNxgDM5ETMwADMwADMwADMwADMxAzL1EzL0gzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
因此,如果對於已給的方程(5)能找到使它不變的變換群(2),就可以取(1)的前一個首次積分中的G1(x,y)=u以代替y而使(5)成為可積方程。例如方程
(11)
![常微分方程變換群理論](/img/5/1b8/ml2ZuM3XxQzM5cTM3UTNxgDM5ETMwADMwADMwADMwADMxAzL1EzLxQzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![常微分方程變換群理論](/img/0/e64/ml2ZuM3X3AzMxgTM3UTNxgDM5ETMwADMwADMwADMwADMxAzL1EzL3AzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![常微分方程變換群理論](/img/5/7dd/ml2ZuM3X5cTNygTM3UTNxgDM5ETMwADMwADMwADMwADMxAzL1EzL5czLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
以上的方法也可用於高階方程的降階,例如方程
(12)
![常微分方程變換群理論](/img/3/5b0/ml2ZuM3XxAzN1gTM3UTNxgDM5ETMwADMwADMwADMwADMxAzL1EzLxAzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![常微分方程變換群理論](/img/5/fed/ml2ZuM3XzYzN2gTM3UTNxgDM5ETMwADMwADMwADMwADMxAzL1EzLzYzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![常微分方程變換群理論](/img/3/330/ml2ZuM3X0cDO3gTM3UTNxgDM5ETMwADMwADMwADMwADMxAzL1EzL0czLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
此外,值得一提的是M.S.李、(C.-)É.皮卡等將變換群理論用於線性變係數齊次方程
參考書目
J. M. Hill, Solution of Differential Equations by Means of One Parameter Groups,Research Notes in Math., 63, 1982.