# parametrically forced pendulum
# forcpend.ode
yo(t)=a*sin(w*t)
yopp(t)=-a*w*w*sin(w*t)
th'=thp
thp'=(-f*thp-m*g*sin(th))/(m*l*l)-yopp(t)*sin(th)/l
par l=2,g=9.8,m=1
par a=0.3,w=50,f=.25
init th=2
@ meth=qualrk
@ total=100,bound=10000,tol=1e-5,atol=1e-4
done