/*


(d4) This program computes a perturbation solution for


the periodic response of Duffing's equation.


Call it by typing:


                     DUFFING(N)


where N is the order of truncation.
*/


duffing(n):=(setup1(n),setup2(n),
	for i thru n do
	    (step1(i),step2(i),step3(i),step4(i),step5(i),output1(i)),
	output2(n))$
setup1(n):=(w:1,for i thru n do w:w+k[i]*e^i,x:a*cos(t),
       for i thru n do x:x+y[i](t)*e^i)$
setup2(n):=(temp1:diff(x,t,2)+x/w^2+e*x^3/w^2-e*f*cos(t)/w^2,
       temp1:taylor(temp1,e,0,n),for i thru n do eq[i]:coeff(temp1,e,i))$
step1(i):=temp1
      :expand(trigreduce(expand(ev(eq[i],makelist([e[j],f[j]],j,1,i-1),
				   diff))))$
step2(i):=(f[i]:solve(coeff(temp1,cos(t)),k[i]),temp1:ev(temp1,f[i]))$
step3(i):=temp1:ev(ode2(temp1,y[i](t),t),%k1:a[i],%k2:b[i])$
step4(i):=(temp2:rhs(temp1),temp2:diff(temp2,t),
      temp2:solve([ev(rhs(temp1),t:0),ev(temp2,t:0)],[a[i],b[i]]))$
step5(i):=e[i]:ev(temp1,temp2)$
output1(i):=(print(expand(e[i])),print(" "))$
output2(n):=(print("w=",ev(w,makelist([f[j]],j,1,n))),print(" "))$