Try to work out the Laplace inverse of 1/s e^(-st_0−sb)

Owen Mathis

Owen Mathis

Answered question

2022-11-17

I am trying to work out the Laplace inverse of 1 s e s t 0 s b , I was trying to use the second shift theorem and rewrote it as e s t 0 1 s e s b .
I ended up with H ( t t 0 ) H ( t b t 0 ), but have been told the solution is H ( t b T 0 ), what happens to the other heaviside unit function? Is there a general rule as to why this happens or have I just worked out the transform wrong?

Answer & Explanation

Kayleigh Cross

Kayleigh Cross

Beginner2022-11-18Added 19 answers

If b>0 then:
H ( t t 0 ) H ( t b t 0 ) = H ( t b t 0 )
indeed:
If t < t 0 then both t t 0 and t b t 0 are negative so:
H ( t t 0 ) H ( t b t 0 ) = 0 × 0 = 0 = H ( t b t 0 )
If t 0 t < t 0 + b then both t t 0 0 and t b t < 00 :
H ( t t 0 ) H ( t b t 0 ) = 1 × 0 = 0 = H ( t b t 0 )
If t t 0 + b then both t t 0 and t b t 0 are postive so:
H ( t t 0 ) H ( t b t 0 ) = 1 × 1 = 1 = H ( t b t 0 )
The general rule is that, by denoting χ A the characteristic function of a set A:
χ A χ B = χ A B
Here:
H ( t τ ) = χ [ τ , + )
and:
[ τ 1 , + ) [ τ 2 , + ) = [ max ( τ 1 , τ 2 ) , + )

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