Subscript ‘dT’ as well. The code will be more efficient if you move the plot outside the loop:
t = [0, 5]; % time span
y0 = 1; % initial condition
dy0 = 2; % initial condition first derivative
dydt = @(t,y) [y(2); ((1 - y(1)^2))*y(2) - y(1)];
for j = 1:10
TOL(j) = 1*exp(-1-j);
options = odeset('AbsTol',TOL(j),'RelTol',TOL(j));
[T{j},Y{j}] = ode45(dydt, t, [y0, dy0], options);
dT(j) = mean(1/length(T)); % average step size
end
figure
plot(log(dT),log(TOL));
grid
