function dzdt=fp2eq(t,z,a,c,f,omega)
%FP2EQ Forced pendulum equations (version 2).
%   DZDT=FP2EQ(T,Z,A,C,F,OMEGA) evaluates
%  
%     dx/dt = y
%     dy/dt = -(A^2 + F*cos(OMEGA*t))*sin(x) - C*y
%
%   at (x,y)=Z and t=T.

%   Francisco J. Beron-Vera, 2006/10/09

n=length(z);
dzdt=zeros(n,1);
x=z(1:2:end-1);
y=z(2:2:end);
dzdt=[y;-(a^2+f*cos(omega*t))*sin(x)-c*y];
dzdt=reshape(dzdt,[n/2 2])';
dzdt=dzdt(:);