function vf = vfield3(t,vec)
% Defines the vector field in computing the bifurcation curves for f=func3
% vf(1) is df/dx, vf(2) is -df/da
global b
a = vec(1);
x = vec(2);

vf(1) = a - 3*x.^2;
vf(2) = -x;
vf = vf';