function y = rk_2d(x,t,dt)


k1=function_2d(x,t);
k2=function_2d(x+k1*dt/2,t+dt/2);
k3=function_2d(x+k2*dt/2,t+dt/2);
k4=function_2d(x+k3*dt,t+dt);


y=x+dt*(k1+2*k2+2*k3+k4)/6;


end