# How to call an ode solver within another function?

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L'O.G. am 5 Okt. 2023
Beantwortet: Sam Chak am 5 Okt. 2023
I solve dx/dt = x(t) using an ode solver
sol = ode45(@(t,x) u(t,x,ode,eps,a0),[t0 tf],[x1;x2]);
that calls a function u for the velocity
function v = u(t,x,ode,eps,a0)
deltaX = x2 - x1; % they are different at each time point: x2(t) and x1(t)
[~,at] = ode45(@(t,a_t) ode(t,a_t,deltaX),[0 t + eps],a0);
a_t = interp1(linspace(0, t + eps, numel(at)), at, t, 'linear');
% ... some calculation involving a_t
end
where I wish to solve the differential equation:
ode = @(t,a_t,deltaX) a0 * (x2-x1) / a_t);
which gives da/dt for a(t) for each time point since x2 and x1 are updated at each time point.
Since I get an error message saying the last entry in tspan must be different from the first entry, I introduced a small positive number eps and then interpolate for the given t, but this interpolated value a_t changes depending on the tspan value, so I don't believe I did this correctly.
In other words, I am trying to solve for x(t) given the velocity that itself depends on a term a(t) that depends on deltaX = x2(t)-x1(t). I feel like the solver for a(t) needs to be within the function u since a(t) is required for other calculations at a given time. How do I do this?
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Walter Roberson am 5 Okt. 2023
[~,at] = ode45(@(t,a_t) ode(t,a_t,deltaX),[0 t + eps],a0);
a_t = interp1(linspace(0, t + eps, numel(at)), at, t, 'linear');
That code is wrong. Use
tspan = linspace(0, t+eps, 5);
sol = ode45(@(t,a_t) ode(t,a_t,deltaX), tspan, a0);
a_t = deval(sol, t);
L'O.G. am 5 Okt. 2023
Thanks. I get the same result. Does it make sense to have the ode solver for a(t) nested in the other function, and for tspan to go from 0 to t+eps?

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### Akzeptierte Antwort

Sam Chak am 5 Okt. 2023
I'm uncertain if I've correctly deduced the mathematical problem from your code. However, a singularity will occur if due to a division-by-zero term in . Since these are coupled ordinary differential equations (ODEs), if the integrations of occur at the same time interval, you can include them in the same odefcn() function, as demonstrated below:
t0 = 0;
tf = 400;
tspan = [t0 tf];
a0 = 0.1;
x0 = [0 1 a0];
[t, x] = ode45(@(t, x) odefcn(t, x, a0), tspan, x0);
plot(t, x), grid on
xlabel({'$t$'}, 'interpreter', 'latex', fontsize=14),
ylabel({'$\mathbf{x}(t)$'}, 'interpreter', 'latex', fontsize=14)
legend({'$x_{1}(t)$', '$x_{2}(t)$', '$a_{t}$'}, 'interpreter', 'latex', fontsize=16)
function dxdt = odefcn(t, x, a0)
dxdt = zeros(3, 1);
at = x(3);
xref = 0.25*sin(pi/5*t) + 0.1;
dxdt(1) = x(2) - x(1); % sample ode 1
dxdt(2) = - 0.1*(x(1) - xref) - 0.2*at*tanh((x(2) - x(1))/0.01); % sample ode 2
dxdt(3) = a0*(x(2) - x(1))/at; % singularity happens when at = 0 % user-supplied ode
end
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