The Role of Complex Exponentials in Laplace Transforms

Complex Integrals and the Laplace Transform There are a few computations for which the complex exponential is very nice to use. We’ll see a few here, but first a couple of Theorems about integrating a complex function: Theorem: Proof: Z Z e(bi)t dt = e(bi)t dt = Z 1 (bi)t e bi e(bt)i dt = Z cos(bt) + i sin(bt) dt = Z cos(bt) dt + i Z sin(bt) dt = 1 i sin(bt) − i cos(bt) sin(bt) − cos(bt) = b b b And 1 (bt)i cos(bt) + i sin(bt) i − sin(bt) + i cos(bt) sin(bt) − i cos(bt) e = · = = bi bi i −b b Therefore, these quantities are the same. 1 e(a+bi)t (a + bi) You can work this out, but it is more complicated since we’ll need to do integration by parts twice for each integral. It is a nice exercise to try out when you have a little time. Theorem: Z e(a+bi)t dt = Theorem: The main computational technique is using the following: 1 e(a+ib)t e cos(bt) dt = Re e dt = Re a + ib Z    Z 1 (a+ib)t at (a+bi)t e e sin(bt) dt = Im e dt = Im a + ib Z Z at (a+bi)t    Worked Example: 1. Use complex exponentials to compute Z e2t cos(3t) dt. SOLUTION: We note that e2t cos(3t) = Re(e(2+3i)t ), so: 1 e(2+3i)t 2 + 3i Simplifying the term inside the parentheses and multiplying out the complex terms: Z e2t cos(3t) dt = Re e2t e 2t     2 − 3i (cos(3t) + i sin(3t)) = 4+9  2 3 3 2 cos(3t) + sin(3t) + i − cos(3t) + sin(3t) 13 13 13 13    Therefore, Z e2t cos(3t) dt = e2t  1 3 2 cos(3t) + sin(3t) 13 13  In fact, we get the other integral for free: Z 2t 2t  e sin(3t) dt = e 2. Use complex exponentials to compute Z −3 2 cos(3t) + sin(3t) 13 13  sin(at) dt This one is simple enough to do without using complex exponentials, but it does still work.   Z  Z 1 (at)i e dt = Im sin(at) dt = Im (cos(at) + i sin(at) = ai      1 −1 −1 −i (cos(at) + i sin(at)) = Im sin(at) + i cos(at) = cos(at) Im a a a a 3. Use complex exponentials to compute the Laplace transform of cos(at): SOLUTION: Note that cos(at) = Re(e(at)i ) L(cos(at)) = Z ∞ e −st Z ∞ Z ∞ e cos(at) dt = Re e  dt = 0 0 Re −st (ai)t  e−(s−ai)t dt = Re 0 −1 e−(s−ai)t (s − ai) t→∞ t=0 What happens to our expression as t → ∞? The easiest way to take the limit is to check the magnitude (see if it is going to zero): −1 −st (ai)t −1 = · e−st · e(ai)t e e s − ai s − ai Now, the first term is a constant and e(at)i is a point on the unit circle (so its magnitude is 1). Therefore, the magnitude depends solely on e−st , where s is any real number. And, the function e−st → 0 as t → ∞ for any s > 0. Therefore, −1 e−(s−ai)t = 0 t→∞ (s − ai) lim and the Laplace transform is:  L(cos(at)) = Re 0 − −1 s + ai = Re 2 s − ai s + a2    = s2 s + a2 As a side remark, we get the Laplace transform of sin(at) for free since it is the imaginary part. 2 Homework Addition to Section 6.1 1. Use complex exponentials to compute Z e−2t sin(3t) dt. 2. Use complex exponentials to compute the Laplace transform of sin(at). 3. Use complex exponentials to compute the Laplace transform of eat sin(bt) and eat cos(bt) (compare to exercises 13, 14). 4. Prove that et goes to infinity faster than any polynomial. You can do that by showing tn =0 t→∞ et lim 5. We can show that f (x) < g(x) for all x ≥ a by proving two things: (i) f (a) < g(a), and (ii) f 0 (x) < g 0 (x) for all x > a. Use this idea to prove that ln(t) < t for all t ≥ 1 (it is true for all t > 0, but we wouldn’t be able to use this argument for 0 < t < 1). 6. Show that, if f (t) is bounded (that is, there is a constant A so that |f (t)| ≤ A for all t), then f is of exponential order (do this by finding K, a and M from the definition). 7. If the function is of exponential order, find the K, a and M from the definition. Otherwise, state that it is not of exponential order. Something that may be handy from algebra: A = eln(A) . 2 (a) sin(t) (d) et (b) tan(t) (e) 5t (c) t3 (f) tt 8. Use complex exponentials to find the Laplace transform of t sin(at). 3