function dxdt = oscillator1(t,x)

a = 0;
k = 1;
k3 = 1;
m = 1;



dxdt(1) = x(2);
dxdt(2) = -(1/m)*(k*x(1) + k3*x(1)^3)- a*x(2);

dxdt = dxdt';