function dxdt = fpuani1(t,x)

a = 0;
k = 1;
k3 = 1;
m = 1;
%D =0.014;

N = length(x)/2; 


y = sqrt(2/3)*atan(exp(0.1*sqrt(6)*(t-50)));

dxdt(1) = x(N+1);
dxdt(N+1) = 1/m*(k*(x(2) - x(1)) + k3*(x(2) - x(1))^3 - k*(x(1) - y) ...
    - k3*(x(1) - y)^3 - a*x(N+1));
for n = 2:(N-1)
   dxdt(n) = x(N+n);
   dxdt(N+n) = 1/m*(k*(x(n+1) - x(n)) + k3*(x(n+1) - x(n))^3 ...
        + k*(x(n-1) - x(n)) + k3*(x(n-1) - x(n))^3 - a*x(N+n));
end
dxdt(N) = x(2*N);
dxdt(2*N) = 1/m*(- k*(x(N) - x(N-1)) - k3*(x(N) - x(N-1))^3 - a*x(2*N));
dxdt = dxdt';