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confusion on a new schema for solving ode

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汉武 沈
汉武 沈 on 15 Jan 2021
Commented: 汉武 沈 on 15 Jan 2021
Hello all, recently i've been working on an algorithm for solving ode, called QSS family, which was found by Prof.Ernesto Kofman. I want to run the algorithm on matlab code, but i'm not sure if the code is right. Can anyone help me to tell if it's right?
The code below is an example to solve the ode:, with and all equal 0 at .
clc
clear
tic
delta_q=1e-4;
q1=0;q2=0;
x1=0;x2=0;
t=0;delta_t=0;tfinal=20;
A=zeros(0,3);
n=0;
while (t<tfinal)
Dx1=q2;
Dx2=1-3*q1-4*q2;
if (Dx1>0)
delta_x1=delta_q+q1-x1;
else
delta_x1=delta_q-q1+x1;
end
if (Dx2>0)
delta_x2=delta_q+q2-x2;
else
delta_x2=delta_q-q2+x2;
end
delta_t1=delta_x1/abs(Dx1);
delta_t2=delta_x2/abs(Dx2);
if (delta_t1<delta_t2)
delta_t=delta_t1;
t=t+delta_t;
x1=x1+Dx1*delta_t;
x2=x2+Dx2*delta_t;
q1=x1;
else
delta_t=delta_t2;
t=t+delta_t;
x1=x1+Dx1*delta_t;
x2=x2+Dx2*delta_t;
q2=x2;
end
n=n+1;
A(n,1)=t;
A(n,2)=x1;
A(n,3)=x2;
% A(n,4)=q1;
% A(n,5)=q2;
end
A;
figure(1)
plot(A(:,1),A(:,2),A(:,1),A(:,3));
toc

Accepted Answer

Bobby Fischer
Bobby Fischer on 15 Jan 2021
Hello,
function euler1
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
clc
clear
tic
delta_q=1e-4;
q1=0;q2=0;
x1=0;x2=0;
t=0;delta_t=0;tfinal=20;
A=zeros(0,3);
n=0;
while (t<tfinal)
Dx1=q2;
Dx2=1-3*q1-4*q2;
if (Dx1>0)
delta_x1=delta_q+q1-x1;
else
delta_x1=delta_q-q1+x1;
end
if (Dx2>0)
delta_x2=delta_q+q2-x2;
else
delta_x2=delta_q-q2+x2;
end
delta_t1=delta_x1/abs(Dx1);
delta_t2=delta_x2/abs(Dx2);
if (delta_t1<delta_t2)
delta_t=delta_t1;
t=t+delta_t;
x1=x1+Dx1*delta_t;
x2=x2+Dx2*delta_t;
q1=x1;
else
delta_t=delta_t2;
t=t+delta_t;
x1=x1+Dx1*delta_t;
x2=x2+Dx2*delta_t;
q2=x2;
end
n=n+1;
A(n,1)=t;
A(n,2)=x1;
A(n,3)=x2;
% A(n,4)=q1;
% A(n,5)=q2;
end
figure(1)
close(1)
figure(1)
subplot(1,3,1)
hold on
axis([0,20 -0.05 0.35])
plot(A(:,1),A(:,2),'b')
plot(A(:,1),A(:,3),'r');
grid on
title('HS')
toc
whos
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
t=linspace(0,20,501);
x0=[0 0];
whos
[t,x]=ode45(@fun,t,x0);
whos
subplot(1,3,2)
hold on
axis([0,20 -0.05 0.35])
plot(t,x(:,1),'k')
plot(t,x(:,2),'k')
grid on
title('edo45')
subplot(1,3,3)
hold on
axis([0,20 -0.05 0.35])
plot(A(:,1),A(:,2),'b')
plot(A(:,1),A(:,3),'r');
plot(t,x(:,1),'k.')
plot(t,x(:,2),'k.')
grid on
title('HS, edo45')
function [dxdt]=fun(~,x)
x1p=x(2);
x2p=1-3*x(1)-4*x(2);
dxdt=[x1p; x2p];
end
end
  1 Comment
汉武 沈
汉武 沈 on 15 Jan 2021
Thank you guy! In fact, my original intention was to let others to help me code the algorithm more effeciently, then i felt the code is okay for now. Thank you again anyway!
One more thing, I have upated another question about ode, hope you can help me!

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