how to solve non linear simultaneous ordinary differential equation?

= (35)(y − x)

= (-7)x − xz + (28)y

= xy − (2.97)z

I solved this problem using ode23 like this-

function dydt = odefcn(t,y)
dydt = zeros(3,1);
dydt(1) = 35*y(2)- 35*y(1)
dydt(2) = (-7)*y(1)-y(1)*y(3)+28*y(1)
dydt(3) = y(1)*y(2)-(2.97)*y(3)
tspan = [0 5]
y0 = [1 0 1]
[t,y] = ode23(@(t,y) odefcn(t,y), tspan, y0)

The error that i am getting is-

Not enough input arguments.

Error in odefcn (line 3)

dydt(1) = 35*y(2)- 35*y(1)

Is it the right way of solving above problem?

Answer 1

[Jan]

How did you call the function? Using the green triangle in the editor? Then no inputs are provided.

The posted code consists of two parts, but it looks, like you have inserted in in one m-file. A sorted version:

function main
tspan = [0 5];
y0 = [1 0 1];
[t, y] = ode23(@odefcn, tspan, y0);
plot(t, y);
end

function dydt = odefcn(t,y)
dydt    = zeros(3,1);
dydt(1) = 35 * y(2) - 35 * y(1);
dydt(2) = -7 * y(1) - y(1) * y(3) + 28 * y(1);
dydt(3) = y(1) * y(2) - 2.97 * y(3);
end

To improve the readability I've inserted spaces around the operators.

@(t,y) odefcn(t,y) can be abbreviated to @odefcn.

[Meenakshi Tripathi]

Thanks a lot!!

Seems you are too good in matlab coding. I am a beginner so it is getting tough for me to do coding.

How to be good in matlab coding related to ode?

Please Help!!

[Jan]

Experiences are useful. Try it, try it again, and try it another time. :-) I'm doing this for some decades now.

[Meenakshi Tripathi]

ok Thank you !!

[Fadzai Zonke]

Hi Jan,

Could you please help me solve 4 non linear ODEs simultaneosly?

[Jan]

@Fadzai Zonke: Maybe. Do you mean this question: https://www.mathworks.com/matlabcentral/answers/1464154-could-someone-please-help-me-fix-the-errors-in-my-code-i-am-trying-to-solve-4-ode ?

[Fadzai Zonke]

It is a different question, somthing similiar

I have 4 non linear differential equations:

for example

dA/dt = 7 - 3*A + 4*T*A

dB/dt = 9 - 4*B + 2*B/A - 5*D*(2-B)

dC/dt = 2- C + 3*B/4

dD/dt = 6 - A/4 - D/3

t = linspace[0 300]

I need to solve them and plot a grapgh for each Variable against t (A,t),(B,t),(C,t),(D,t)

[Jan]

You find an example for implementing your equation in Matlab. The elements of y are [A,B,C,D] in your case.