高阶线性微分方程学习笔记
相关定义:
- 高阶线性微分方程一般形式:
p0(t)x(n)(t)+p1(t)x(n−1)(t)+...+pn−1(t)x′(t)+pn(t)x(t)=f(t)若p0(t)不等于0,上式可写成:x(n)+P1(t)x(n−1)+...+Pn−1(t)x′+Pn(t)x=F(x)其中Pi(t)=pi(t)p0(t). p_0(t)x^{(n)}(t)+p_1(t)x^{(n-1)}(t)+...+p_{n-1}(t)x'(t)+p_n(t)x(t)=f(t)\\ 若p_0(t)不等于0,上式可写成:\\ x^{(n)}+P_1(t)x^{(n-1)}+...+P_{n-1}(t)x'+P_n(t)x=F(x)\\ 其中P_i(t)=\frac{p_i(t)}{p_0(t)}. p0(t)x(n)(t)+p1(t)x(n−1)(t)+...+pn−1(t)x′(t)+pn(t)x(t)=f(t)若p0(t)不等于0,上式可写成:x(n)+P1(t)x(n−1)+...+Pn−1(t)x′+Pn(t)x=F(x)其中Pi(t)=p0(t)pi(t).
为书写简洁,引入记号
L(x)=x(n)+P1(t)x(n−1)+...+Pn−1(t)x′+Pn(t)x L(x)=x^{(n)}+P_1(t)x^{(n-1)}+...+P_{n-1}(t)x'+P_n(t)x L(x)=x(n)+P1(t)x(n−1)+...+Pn−1(t)x′+Pn(t)x
从而线性齐次方程可简洁的写为
L(x)=0L(x)=0L(x)=0,
其中,
L()=dndtn+P1(t)dn−1dtn−1+...+Pn−1(t)ddt+Pn(t)L()=\frac{d^n}{dt^n}+P_1(t)\frac{d^{n-1}}{dt^{n-1}}+...+P_{n-1}(t)\frac{d}{dt}+P_n(t)L()=dt