# Thread: Implicit Euler Scheme and stability

1. ## Implicit Euler Scheme and stability

Find the fixed points of the implicit Euler scheme
y_{n+1}-y_{n}= hf(t_{n+1},y_{n+1})

when applied to the differential equation $y'=y(1-y)$ and investigate their stability?
=>
implicit Euler scheme
y_{n+1}-y_{n}= hf(t_{n+1},y_{n+1})

$y'=y(1-y)$
y_{n+1}=y_{n}+hy_{n+1}(1-y_{n+1})

y_{n+1}=y_{n}+hy_{n+1}-hy^2_{n+1}

For fixed points
$y_{n+1}=y_{n}$
y_{n}=y_{n}+hy_{n}-hy^2_{n}

$y_{n}=0$ or $1$
I got problem with stability but this is what I have done
$y_{n}= \alpha +\epsilon^n$, $y_{n+1}= \alpha +\epsilon^{n+1}$,
$$\alpha +\epsilon^{n+1}= \alpha +\epsilon^n + h (\alpha +\epsilon^{n+1})(1-\alpha -\epsilon^{n+1})$$
$$\epsilon^{n+1}= \epsilon^n + h (\alpha +\epsilon^{n+1})(1-\alpha -\epsilon^{n+1})$$
When $y_{n}=0=\alpha$
$$\epsilon^{n+1}= \epsilon^n + h \epsilon^{n+1}(1-\epsilon^{n+1})$$
I don't what to say or do after that to determine the stability.
When $y_{n}=1=\alpha$
$$\epsilon^{n+1}= \epsilon^n - h \epsilon^{n+1}(1+\epsilon^{n+1})$$
same again what can say about with my answer to investigate the stability.

2. ## Re: Implicit Euler Scheme and stability

what makes a stationary point stable vs. unstable? (hint think 2nd derivative)

3. ## Re: Implicit Euler Scheme and stability

h>0 unstable
h<0 stable
but how to get into that form. that's where I got stuck.