It looks like F is not smooth. Then Matlab's integrators cannot handle this directly, see https://www.mathworks.com/matlabcentral/answers/59582-ode45-errer-input-argument-y-is-undefined#answer_72047.
The best way is a loop:
t = linspace(3.6,7.2,81); %time vector
F = 1:1:81; % Right hand side
x0 = [0, 0];
t0 = t(1);
for iStep = 2:numel(t)
Fs = F(iStep - 1); % For example
[ts, xs] = ode45(@(t, x) funct(t, x, Fs), [t0, t(iStep)], x0);
t0 = ts;
x0 = xs(end, :);
% Store ts and xs for the output...
end
function dxdt = funct(t,x, F)
dxdt = [x(2); ...
F - (2*pi/t)^2*x(1)];
end
This is untested and thought to show the concept only.
