I'd like help with the ode45 function for a biological model

I'm trying to solve a differential equation using the ode45 model however I am encountering a recurring error problem here is my script:

function dydt = odepharm(t,y,S,R)

dydt = zeros(2,1);

dydt(1) = (gs*(Nmax-(y(1)+y(2))/Nmax)-ks*c/c+EC50s)*y(1);%S;

dydt(2) = (gr*(Nmax-(y(1)+y(2))/Nmax)-kr*c/c+EC50r)*y(2);%R;

end

gs=0.99;

gr=0.66;

Nmax=1.10e9;

ks=1.56;

kr=1.17;

c=400;

EC50s=0.21;

EC50r=5.19;

tspan = [0:12:96];

y0 = [1.06 1.06];

[t,y] = ode45(@(t,y) odefpharm(t,y,S,R), tspan, y0);

The error message includes:

Unrecognized function or variable 'S'.

Error in @(t,y)odefpharm(t,y,S,R)

Error in odearguments (line 92)

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

Error in ode45 (line 107)

odearguments(odeIsFuncHandle,odeTreatAsMFile, solver_name, ode, tspan, y0, options, varargin);

Answer 1

[Sam Chak]

Hi @Chatowa

Try if you are expecting this result for

tspan  = [0 96];
y0     = [1.06 1.06];
[t, y] = ode15s(@odepharm, tspan, y0);
plot(t, y(:,1)), grid on, xlabel('t'), ylabel('y_{1}')

function dydt = odepharm(t,y)
    dydt    = zeros(2,1);
    gs      = 0.99;
    gr      = 0.66;
    Nmax    = 1.10e9;
    ks      = 1.56;
    kr      = 1.17;
    c       = 400;
    EC50s   = 0.21;
    EC50r   = 5.19;
    dydt(1) = (gs*(Nmax - (y(1) + y(2))/Nmax) - ks*c/c + EC50s)*y(1); % S;
    dydt(2) = (gr*(Nmax - (y(1) + y(2))/Nmax) - kr*c/c + EC50r)*y(2); % R;
end