What did I do wrong here?

I am trying to calculate the amount of fish I can catch in a year based on this model. What did I do wrong? The answer that I got is super huge.

u0 = 5000; 
lambda = 0.03;
pm = 9000;
f = @(t,k) lambda*500*(1-500/pm)-k;
[t,k] = RK4(f,u0,100,10)
Kf = k(end)

[Image Analyst]

What I get is

Unrecognized function or variable 'RK4'.

Where did you define that?

[Zifeng Qiu]

Sorry, that was a function that I wrote, here are the code:

function [tv, uv] = RK4(f, u0, T, n)
dt = T/n; % Differential of time.
tv = transpose(0:dt:T); % Evaluation times.
uv = zeros(n+1, 1); % Solution.

uv(1) = u0; % Initial value
for i=1:length(tv)-1
    k1 = f(tv(i), uv(i))
    k2 = f(tv(i) + (dt/2), uv(i) + dt*k1/2)
    k3 = f(tv(i) + (dt/2), uv(i) + dt*k2/2)
    k4 = f(tv(i) + dt, uv(i) + dt*k3)
    uv(i+1) = uv(i) + dt/6*[k1+2*k2+2*k3+k4]
end
end

Answer 1

[Alan Stevens]

I answered this here:

https://uk.mathworks.com/matlabcentral/answers/559760-how-to-do-this-problem#answer_461267?s_tid=prof_contriblnk