# Forward Euler solution plotting for dy/dt=y^2-y^3

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David am 15 Nov. 2022
Kommentiert: David am 23 Nov. 2022
Hi,
I am trying to solve the flame propagation model dy/dt=y^2-y^3 with y(0)= 1/100 and 0<t<200, using the forward and backward euler method with step size 0.01. But it has been giving me errors. How should I go about this? Please help I need this for my project
Thank you
Here are my codes for the Forward Euler
h=0.01;
y(0)=2
for n=1:N
t(n+1)=n*h
opts = odeset('RelTol',1.e-4);
y(n+1)= y(n)+h*(y.^2-y.^3);
end
plot(t,y)
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### Akzeptierte Antwort

Davide Masiello am 15 Nov. 2022
Bearbeitet: Davide Masiello am 15 Nov. 2022
There are a couple of issues with your code
1) Indexes in MatLab start at 1, not 0, so y(0) is not valid syntax and must be replaced with y(1).
2) First index, then raise to the power, i.e. y^2(n) becomes y(n)^2
See example below (since you have not specified the value of delta, I arbitrarily replaced it with 0.1)
N = 100; % number of steps
t = linspace(0,200,N);
h = t(2)-t(1); % step size
y = zeros(1,N); % Initialization of solution (speeds up code)
y(1) = 1/100; % Initial condition
for n = 1:N-1
y(n+1) = y(n)+h*(y(n)^2-y(n)^3); % FWD Euler solved for y(n+1)
end
figure(1)
plot(t,y)
Now, the backward euler method is a bit more complicated because it's an implicit method.
You can use a root finder algorithm like fzero and do
N = 100; % number of steps
t = linspace(0,200,N);
h = t(2)-t(1); % step size
y = zeros(1,N); % Initialization of solution (speeds up code)
y(1) = 1/100; % Initial condition
for n = 1:N-1
f = @(x) x-y(n)-h*(x.^2-x.^3);
y(n+1) = fzero(f,y(n)); % BWD Euler solved for y(n+1)
end
figure(2)
plot(t,y)
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David am 23 Nov. 2022
Thank you very much, this helped me so much

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