Laplace Transformation

1. Basic notions
Definition
For any complex valued function f defined for t > 0 and complex number s, one defines
the Laplace transform of f (t) by
Z ∞
F (s) = e−st f (t) dt,
0
if the above improper integral converges.

Notation
We use L(f (t)) to denote the Laplace transform of f (t).

Remark
It is clear that Laplace transformation is a linear operation: for any constants a and b :
L(af (t) + bg(t)) = aL(f (t)) + bL(g(t)).

Remark
It is evident that F (s) may exist for certain values of s only. For instance, if f (t) = t, the
Laplace transform of f (t) is given by (using integration by parts : u $ t, v 0 $ e−st ):
Z Z
t −st 1 t 1
−st
te dt = − e − (− e−st dt) = − e−st + 2 e−st ;
Z ∞ s s ∞ s s
t 1 1
; te−st dt = − e−st + 2 e−st  = 2 if s > 0 and does not exists if s ≤ 0 .

0 s s 0 s
1
Therefore L(t) = .
s2
Theorem
If f (t) is a piecewise continuous function defined for t ≥ 0 and satisfies the inequality
|f (t)| ≤ M ept for all t ≥ 0 and for some real constants p and M , then the Laplace
transform Lf (t)) is well defined for all Re s > p.

Illustration
The function f (t) = e3t has Laplace transform defined for any Re s > 3, while g(t) = sin kt
has Laplace transform defined for any Re s > 0. The tables on the following page give the
Laplace transforms of some elementary functions.

Remark
It is clear from the definition of Laplace Transform that if f (t) = g(t) , for t ≥ 0, then
F (s) = G(s). For instance, if H(t) is the unit step function defined in the following way:
1
H(t) = 0 if t < 0 and H(t) = 1 if t ≥ 0 , then L(H(t)) = L(1) = and (as we have
s
1 n!
seen above) L(H(t)t) = L(t) = 2 . Generally, L(H(t)tn ) = L(tn ) = n+1 .
s s

1
2. Inverse Laplace Transforms
Definition
If, for a given function F (s), we can find a function f (t) such that L(f (t)) = F (s), then
f (t) is called the inverse Laplace transform of F (s). Notation: f (t) = L−1 (F (s)).

Examples
   
−1 1 −1 1 sin ω t ω
L = t. L = (hiszen L(sin ω t) = és L lineáris.)
s2 s2 + ω 2 ω s2 + ω 2

We are not going to give you an explicit formula for computing the inverse Laplace
Transform of a given function of s. Instead, numerous examples will be given to show
how L−1 (F (s)) may be evaluated. It turns out that with the aide of a table and some
techniques from elementary algebra, we are able to find L−1 (F (s)) for a large number of
functions.
Our first example illustrates the usefulness of the decomposition to partial fractions:

Example
5s2 + 3s + 1 5s2 + 3s + 1
 
2s − 1 3
2
(s + 1)(s + 2)
= 2 +
s +1 s+2
; L −1
(s2 + 1)(s + 2)
= 2 cos t − sin t + 3e−2t .

3. Some simple properties of Laplace Transform

3.1 Transform of derivatives and integrals
If f and f0 are continuous for t > 0 such that f (t)e−st −→ 0 as t −→ ∞, then we may
integrate by parts to obtain (F (s) = L(f (t)))
Z ∞
0
(1) L(f (t)) = e−st f 0 (t) dt = sF (s) − f (0)
0
Z ∞ ∞
Z ∞
indeed by u 0 = f 0 (t) , v = e−st : f 0 (t)e−st dt = f (t)e−st 0 − s f (t)e−st dt = −f (0) − sF (s)


0 0
and applying this formula again (assuming the apropriate conditions concerning the func-
tion and it first and second derivative hold):
L(f 00 (t)) = sL(f 0 (t)) − f 0 (0) = s(sF (s) − f (0)) − f 0 (0) = s2 F (s) − sf (0) − f 0 (0) .
Similarly (again assuming the apropriate conditions concerning the derivatives hold) we
obtain the general formula:
L(f (n) (t)) = sn F (s) − sn−1 f (0) − sn−2 f 0 (0) − . . . − sf (n−2) (0) − f (n−1) (0) .
Example
L(cos t) = L(sin 0 t) = sL(sin t) − sin 0 = s
s2 +1

It follows from (1) that

1
(*) L(f (t)) = F (s) = (L(f 0 (t)) + f (0))
s
Example
1 2
L(sin2 t) = (L(sin 2t) + 0) = 2
s s(s + 4)

2
Corollary Z t
1
If f is continuous, then L( f (τ )dτ ) = L(f (t)) .
0 s
Z t
(Indeed (*) can be applied to the function g(t) = f (τ ) d τ .)
0

3.2 Transform of shifts in s and t

(a) If L(f (t)) = F (s), then L(eat f (t)) = F (s − a) for any real constant a.
Note that F (s − a) represents a shift of the function F (s) by a units to the right.

(b) The unit step function s(t) = 0 , ha t < 0 és s(t) = 1 , ha t ≥ 0 :

If a > 0 and L(f (t)) = F (s) , then L(f (t − a) · s(t − a)) = F (s)e−as .

Example
Since s2 − 2s + 10 = (s − 1)2 + 9, we have  
s+2 s−1 3 −1 s+2
= + , ı́gy L = et (cos 3t + sin 3t) .
s2 − 2s + 10 (s − 1)2 + 9 (s − 1)2 + 9 s2 − 2s + 10

3.3 Transform of power multipliers

If L(f (t)) = F (s) , then
dn
L(tn f (t)) = (−1)n F (s)
dsn
for any positive integer n , particularly L(tf (t)) = (−1)F 0 (s) .

3.4 Convolution
Definition
Given two functions f and g, we define, for any t > 0 ,
Z t
(f ∗ g)(t) = f (x)g(t − x) dx .
0
The function f ∗ g is called the convolution of f and g.

Remark The convolution is commutative.

Theorem (The convolution theorem)

L((f ∗ g)(t)) = L(f (t)) · L(g(t)) .

In other words, if L(f (t)) = F (s) and L(g(t)) = G(s) , then L−1 (F (s)G(s)) = (f ∗ g)(t) .

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Example
       
−1 s −1 s 1 −1 s −1 1
L =L · =L ∗L =
(s2 + ω 2 )2 s2 + ω 2 s2 + ω 2 s2 + ω 2 s2 + ω 2
1 t
Z
sin ω t
= cos ω t ∗ = cos ω x sin ω(t − x) dx =
 ω ω 0 t
1 1 1 
= 2 cos(−2 ω x + ω t) + t ω sin(ω t)  =
ω 4 2 0
   
1 1 1 1 1 1
= 2 cos(ω t) + t ω sin(ω t) − cos(ω t) = t sin(ω t) ,
ω 4 2 ω 4 2ω
where in order to integrate, we have used addition formulas for the trigonometric functions.
    Z t
−1 1 −1 1 1
A simpler example: L =L = 1 ∗ sin t = sin(t − x) dx =
Z t s(s2 + 1) s s2 + 1 0
t t
= (sin t cos x − cos t sin x)dx = sin t sin x0 + cos t sin x0 = sin2 t + cos2 t − cos t =
0
1 s s2 + 1 − s2 1
= 1 − cos t . Indeed, L(1 − cos t) = − 2 = = .
s s +1 s(s2 + 1) s(s2 + 1)

3.5 Laplace Transform of a periodic function

Definition
A function f is said to be periodic if there is a constant T > 0 such that f (t + T ) = f (t)
for every t . The constant T is called the period of f .
The sine and cosine functions are important examples of periodic function. One other
example is the periodic triangular wave. It is is the function defined by f (t) = t if
0 ≤ t ≤ 1 , f (t) = 2 − t if 1 ≤ t ≤ 2 and f (t + 2) = f (t) for any t .
The following proposition is useful in calculating the Laplace Transform of a periodic
function.

Proposition
Let f be a periodic function with period T and f1 is one period of the function, Then (as
usual F (s) = L(f (t))):
Z T
L(f1 (t)) 1
F (s) = −T s
= −T s
e−st f (t) dt .
1−e 1−e 0

Example
f (t) = 0 ha t < 0 , f (t) = t ha 0 ≤ t ≤ 1 és f (t + n) = f (t) tetszőleges n-re:

1 2

4
Now f1 (t) = 0 if t < 0 and t > 1 , further f1 (t) = t if 0 ≤ t < 1 , then defining
h(t) = 0 if t < 0 and h(t) = t otherwise
g(t) = 0 if t < 0 and g(t) = t + 1 otherwise,

we have f1 (t) = h(t) − g(t − 1) :

h(t)

1 2

g(t−1)

1 2

1
f 1 (t)

1 2

1 − e−s − s e−s
 
1 1 1
Therefore, L(f1 (t)) = L(h(t)) − L(g(t − 1)) = 2 − e−s 2
+ = ,
s s s s2
−s −s
L(f1 (t)) 1 − e − se
that is L(f (t)) = −s
= .
1−e s2 (1 − e−s )