一階方程換元法
尺度變換(Scaling)
x1=ax;y1=by
- 改變單位(change units)
- make vars dimensionally
- redue # , or simplify constants
Direct Sub:
伯努利方程(Bernoulli equation)
y′=p(x)y+q(x)yn(n=0)
齊次(Homogeneous)
一階自治(ODE)
- 找臨界點(Find Critical point)
- 畫圖(Graph)
Logistic EQN: 描述人口增長
Population behavior y(t)
dtdy=ky (k=growth rate)
k constant
k = a- by
dtdy=ay−by2
(應用於疾病傳播,流言傳播)
一階常係數線性方程
General form:
y′+ky=kq(t)
Solution:
y=e−kt∫q(t)ektdt+ce−kt
(ce−kt=0 as t→∞)
穩態解(steady-state solution) ; 長效解(long-term solution)
卷積
convolution
f(t)∗g(t)
F(s)=∫0∞e−stf(t)dt
G(s)=∫0∞e−stg(t)dt
F(s)G(s)=∫0∞e−st(f∗g)dt
F(x)=∑0∞anxn
G(x)=∑0∞bnxn
n→t ; x→e−s
f(t)∗g(t))=∫0tf(u).g(t−u)du