ode45 problem: Index in position 1 is invalid

Hello, I am trying to compute solutions for 2 ODEs for different t values. This always works the first time after I clear the workspace but then I get the following error:

Index in position 1 is invalid. Array indices must be positive integers or logical values.

Error in @(t,y)b(t,y,alfa,sigma)

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);

I have defined the function b as:

function dydt = b(t,y,alfa,sigma)
  dydt = alfa.*y - 0.5.*sigma.^2.*y^2 + 1;
end

and the function a as:

function dydt = a(t,y,alfa,gamma,b)
    dydt = -alfa.*gamma.*b;
end

And then:

T = 30;                
maturities = 0:0.25:T;    
sigma = 0.02;     
gamma = 0.02;      
alfa = 0.1;               

b = deval(ode45(@(t,y) b(t,y,alfa,sigma),[T 0], 0), flip(maturities));
a = deval(ode45(@(t,y) a(t,y,alfa,gamma,deval(ode45(@(t,y) b(t,y,alfa,sigma),[T 0], 0), t)),[T 0], 0), flip(maturities));

How can I solve this error? To which index is Matlab referring when saying it's invalid?

Answer 1

[Walter Roberson]

b = deval(ode45(@(t,y) b(t,y,alfa,sigma),[T 0], 0), flip(maturities));

Before that line, b refers to the function b(t,y,alfa,sigma) that you define.

After that line, b refers to the numeric results of the deval call.

a = deval(ode45(@(t,y) a(t,y,alfa,gamma,deval(ode45(@(t,y) b(t,y,alfa,sigma),[T 0], 0), t)),[T 0], 0), flip(maturities));

There you use b(t,y,alfa,sigma) inside the deval() portion. But b is now an array, not the function.