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Difference equation



finding this pretty difficult

i've learnt difference equation solved using the shift operator E

so i began by rewriting

yt=1.05yt1Dy_{t} = 1.05y_{t-1} - D

as
(E1.05)yt=D(E-1.05)y_{t} = -D

now what?!
(edited 12 years ago)
Reply 1
Original post by Milan.


finding this pretty difficult

i've learnt difference equation solved using the shift operator E

so i began by rewriting

yt=1.05yt1Dy_{t} = 1.05y_{t-1} - D

as
(E1.05)yt=D(E-1.05)y_{t} = -D

now what?!


I don't really know about the shift operator method but what I would do to solve it is first rewrite it as yt=D+1.05yt1y_{t} = - D + 1.05y_{t-1} then replace y_{t-1} with what that is in terms of y_{t-2}, then replace y_{t-2} and so on until you see the general pattern (you might need to put in some dots in the equation when you get y_t in terms of y_0).

From there you'll be able to get the coefficient of D in terms of 1.05, 1.05^2 and so on. Similarly you can get the coefficient of y_0. These can be simplified further with the formula for a geometric series.
Reply 2
Original post by ttoby
I don't really know about the shift operator method but what I would do to solve it is first rewrite it as yt=D+1.05yt1y_{t} = - D + 1.05y_{t-1} then replace y_{t-1} with what that is in terms of y_{t-2}, then replace y_{t-2} and so on until you see the general pattern (you might need to put in some dots in the equation when you get y_t in terms of y_0).

From there you'll be able to get the coefficient of D in terms of 1.05, 1.05^2 and so on. Similarly you can get the coefficient of y_0. These can be simplified further with the formula for a geometric series.


i understand what you mean, but it doesnt help me with answering this particular question :s


can anyone help show me how the answer was obtained... here is the answer:

Reply 3
pleease??:frown:
Reply 4
Original post by Milan.
pleease??:frown:


Sorry, I can't explain the method you've been given because I don't know it myself. What I can do is explain the one I gave you further.

So you have yt=D+1.05yt1=D+1.05(D+1.05yt2)y_{t} = - D + 1.05y_{t-1} = - D + 1.05(-D + 1.05y_{t-2}) =D+1.05(D+1.05(D+1.05(D+1.05yt3)))= - D + 1.05(-D + 1.05(- D + 1.05(-D + 1.05y_{t-3})))

=...=D+1.05(D+1.05(D+1.05(D+1.05(...(D+1.05y0)...))))= ... = - D + 1.05(-D + 1.05(- D + 1.05(-D + 1.05(...(-D +1.05y_0)...))))

If we then multiply out the brackets here, we get yt=D1.05D1.052D...1.05t1D+1.05ty0=1.05ty0Dk=0t11.05ky_t=-D - 1.05D - 1.05^2D - ... - 1.05^{t-1}D + 1.05^ty_0 = 1.05^ty_0 - D \sum_{k=0}^{t-1}1.05^k

From there you can simplify further using the sum of a geometric series, and you can also substitute in what y0y_0 is.


Also by the way, the image you linked to has just gone dead.

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