Download Inverse Laplace Transforms: Finding the Original Function from the Transformed One and more Study notes Differential Equations in PDF only on Docsity!
Differential Equations
LECTURE 25
Inverse Laplace Transforms
At the end of last class, we saw that upon using Laplace transforms on an initial value problem, we end up with an expression for Y (s), the Laplace transform of the desired solution y(t). Thus, we need to know how to go backwards: given a transformed function Y (s), how do we find the original function y(t)? This is a slightly more complicated process than taking transforms, which was quite straightforward. We refer to f (t) as the inverse Laplace transform of F (s) and use the notation f (t) = L−^1 {F (s)}. Our starting point is that the inverse Laplace transform is linear, just like the original transform was.
Theorem 25.1. Given two Laplace transforms F (s) and G(s),
L−^1 {aF (s) + bG(s)} = aL−^1 {F (s)} + bL−^1 {G(s)}
for any constants a, b.
So, we’ll decompose our original transformed function into pieces, inverse transform, and then put everything back together. This is where familiarity with the basic Laplace transforms and the table becomes handy. The strategy is to try to identify the desired inverse transform by looking at the denominator. In most cases, this will tell us what the original function will have to be, but occasionally we will have to look at the numerator to distinguish between two potential inverses (e.g., the denominators for the transforms of sin(at) and cos(at) are the same, but the numerators differ). Then, we know precisely how we have to write our function F (s) so that it is the inverse transform of the function we’ve identified as the inverse. Sometimes this requires a little bit of algebra or arithmetic. Let’s look at some examples.
Example 25.1. Find the inverse transforms of the following.
(i) F (s) =
s
s − 2
s − 3 This one is quite straightforward. The denominator of the first term, (^3) s , indicates that this will be the Laplace transform of 1. Since L { 1 } = (^1) s , we’ll factor out the 3 before taking the inverse transform. For the second term, this is just the Laplace transform of e^2 t, and there’s nothing else to do with it. The third term is also an exponential, e^3 t, and we’ll need to factor out the 4 in the numerator before we inverse transform. So we have
L−^1 {F (s)} = 3L−^1
s
− L−^1
s − 2
f (t) = 3(1) − e^2 t^ + 4
e^3 t
= 3 − e^2 t^ + 4e^3 t.
(ii) G(s) =
s + 3
2 s − 4
s^3
The first term is just the transform of e−^3 t^ multiplied by 5, which we’ll factor out before inverse transforming. The second term looks like it ought to be an exponential, but it’s got a 2s instead of an s in the denominator, and transforms of exponentials should just have s. We can fix this by factoring a 2 out of the denominator and then taking the inverse transform. The third term has s^3 as its denominator. This indicates that it will be related to the transform of t^2. The numerator isn’t quite correct, though, since L
t^2
= (^) s2+12! = (^) s^24. So we would need the numerator to be 2, and right now it’s 7. How do we fix this? We’ll multiply by 22 , absorb the top 2 into the transform, and keep the 72 out front. Let’s start by rewriting the transform, with these fixes incorporated.
G(s) = 5
s − (−3)
2(s − 2)
s^3
= 5
s − (−3)
s − 2
s^3
Now we can take the inverse transform.
g(t) = 5e−^3 t^ −
e^2 t^ +
t^2
(iii) H(s) =
2 s s^2 + 25
s^2 + 16 The denominator of the first term, s^2 + 25, indicates that this should be the transform of either sin(5t) or cos(5t). The numerator is 2s, though, which tells us that once we factor out the 2, it will be the transform of cos(5t). The second term’s denominator is s^2 + 16, so it will be the transform of either sin(4t) or cos(4t). The numerator is a constant, 3, so it will be the transform of sin(4t). The only problem is that the numerator of L {sin(4t)} should be 4, while here it is 3. We’ll fix this, as in the previous example, by multiplying by 44. We rewrite the transform.
H(s) = 2
s s^2 + (5)^2
s^2 + 4^2
= 2
s s^2 + 5^2
s^2 + 4^2 Then we take the inverse.
h(t) = 2 cos(5t) +
sin(4t).
Let’s do some now that require a little more work.
Example 25.2. Find the inverse Laplace transforms for each of the following.
(i) F (s) =
3 s − 7 s^2 + 16 Looking at the denominator, we recognize that this will involve a sine or a cosine, as it has the form s^2 + a^2 .. It’s not quite either, though, since it has both an s and a constant in the numerator, while a cosine just wants the s and the sine just wants the constant.
last couple of examples.
G(s) = − 3
s + 1 (s + 1)^2 + 9
(s + 1)^2 + 9
g(t) = − 3 e−t^ cos (3t) +
e−^2 sin (3t)
(iii) H(s) =
s + 2 s^2 − s − 12 This should appear similar at first glance to the previous example, but there’s a difference: this time, we can factor the denominator. This requires us to deal with the inverse transform differently. Factoring, we see
H(s) =
s + 2 (s + 3)(s − 4)
We know that if we have a linear denominator, that will correspond to an exponential. In this case, we have the product of two linear factors. This by itself isn’t the denominator of any particular Laplace transform, but we know a method for turning certain rational functions with factored denominators into a sum of more simple rational functions with those factors in each denominator: partial fractions. We start by writing
H(s) =
s + 2 (s + 3)(s − 4)
A
s + 3
B
s − 4
We have to put our partial fraction decomposition over a single denominator: s + 2 (s + 3)(s − 4)
A(s − 4) + B(s + 3) (s + 3)(s − 4)
This needs to be true for any value of s; in particular, the numerators must match for every value of s: s + 2 = A(s − 4) + B(s + 3). As a result, we can choose values of s to plug in that will isolate an individual constant. Let’s do this for each.
s = −3 : −1= − 7 A ⇒ A=
s = 4 : 6 = 7B ⇒ B=
Thus, our transform can be written as
H(s) =
1 7 s + 3
6 7 s − 4 and taking the inverse transforms, we get
h(t) =
e−^3 t^ +
e^4 t.
Remark. We could have done the last part of the previous example as we had the previous parts by completing the square. However, this would have left us with expressions involving the hyperbolic sine sinh and the hyperbolic cosine cosh. These are interesting functions which can be written in terms of exponentials (as we got in the form of our answer to the previous example), but it will
Factor in Denominator Partial Fractions Term ax + b
A
ax + b (ax + b)k^
A 1
ax + b
A 2
(ax + b)^2
Ak (ax + b)k ax^2 + bx + c
Ax + B ax^2 + bx + c (ax^2 + bx + c)k^
A 1 x + B 1 ax^2 + bx + c
A 2 x + B 2 (ax^2 + bx + c)^2
Akx + Bk (ax^2 + bx + c)k Table 25.1. Translation from factored denominator to partial fractions.
be much easier for us to work with the exponentials, so we’re better off just doing partial fractions even though it’s slightly more work.
Partial fractions and completing the square are a part of life when it comes to Laplace trans- forms. Being comfortable with these techniques is especially important when we’re working with initial value problems, since most of our answers will involve some combinations of exponentials, sines, and cosines. Let’s quickly review partial fractions. The first step is to factor the denominator as much as you can. Then, using Table 25.1, we can find each of the terms for our partial fractions decomposition. This table isn’t exhaustive, but we’ll only worry about having linear or quadratic factors. Let’s do some more examples that require partial fractions.
Example 25.3. Find the inverse transform of each of the following.
(i) F (s) =
2 s + 1 (s − 2)(s + 3)(s − 1) The form of the decomposition will be
G(s) =
A
s − 2
B
s + 3
C
s − 1 since all of the factors in our denominator are linear. Putting the right hand side over a common denominator and setting numerators equal, we have 2 s + 1 = A(s + 3)(s − 1) + B(s − 2)(s − 1) + C(s − 2)(s + 3). We can once again use the method from the previous example where we choose key values of s that will isolate the coefficients. s = 2 : 5 = A(5)(1) ⇒ A = 1
s = −3 : −5 = B(−5)(−4) ⇒ B = −
s = 1 : 3 = C(−1)(4) ⇒ C = −
Thus, the partial fraction decomposition for this transform is
F (s) =
s − 2
1 4 s + 3
3 4 s − 1
The inverse transform is then
f (t) = e^2 t^ −
e−^3 t^ −
et.
and we have to solve the system of equations
(s^3 ) : A + D = 0 (s^2 ) : −A + B = 0 (s^1 ) : −B + C = 0 (s^0 ) : −C = 2
⇒ A = − 2 B = − 2 C = − 2 D = 2.
Thus our partial fractions decomposition becomes
H(s) = −
s
s^2
s^3
s − 1
= −
s
s^2
s^3
s − 1
and the inverse transform is
h(t) = − 2 − 2 t − t^2 + 2et.
