coding error ode45

clc;
close all;
 
global a b a2 
 
%define parameters
 
%x is the number of cancer cells
x0 = 1000;
 
% c2 is the number of bad stromal cells 
c20 = 30;
 
%b is the number of bad stroma cells (ratio)
b = 20; 
 
% a is the growth rate of cancer cells
a =  0.35;
 
% a2 growth rate of cancerous stroma
 
a2 = 0.4;
 
%time parameter (measured in weeks)
t0=0;           %initital time
t1=520;        %final time 
tspan=t0:1:t1;
 
[t, x] = ode45('badstromarun',tspan,x0); 
figure(1)
 
plot(t,x)
Recall file 
function xdot= badstromarun(t,x)
 
global a b 
xdot = zeros(1,1);
xdot =  (a*x)*(1-log(x)/b)*(c2/(a2+c2));
end

A lot of erros keep occuring

Error in odearguments (line 90)

f0 = feval(ode,t0,y0,args{:}); % ODE15I sets args{1} to yp0.

Error in ode45 (line 115)

odearguments(FcnHandlesUsed, solver_name, ode, tspan, y0, options, varargin);

Error in badstroma (line 29)

[t, x] = ode45('badstromarun',tspan,x0);

Answer 1

[Stephan]

If possible try to avoid global variables:

%define parameters
%x is the number of cancer cells
x0 = 1000;
% c2 is the number of bad stromal cells 
c2 = 30;
%b is the number of bad stroma cells (ratio)
b = 20; 
% a is the growth rate of cancer cells
a =  0.35;
% a2 growth rate of cancerous stroma
a2 = 0.4;
%time parameter (measured in weeks)
t0=0;           %initital time
t1=520;        %final time 
tspan=[t0 t1];
% plot result 
[t, x] = ode45(@(t,x)badstromarun(t,x,a2,a,b,c2),tspan,x0); 
figure(1)
plot(t,x)
% function
function xdot= badstromarun(~,x,a2,a,b,c2)
xdot = zeros(1,1);
xdot(1) =  (a.*x).*(1-log(x)/b)*(c2/(a2+c2));
end