function dY=Pend1(t,Y,const)
% function dY=Pend1(t,Y) is the ODE describing the nonlinear pendulum.
%  const(1)=g (gravity), const(2)=l, length of pendulum.
dY=zeros(size(Y));
dY(1)=Y(2);
dY(2)=-(const(1)/const(2))*sin(Y(1));