Inverse Laplace Transform of 1/s^(a+1) int_0^(oo) e^(-t) x^a. How to make the contour integration.

Leanna Jennings

Leanna Jennings

Answered question

2022-11-16

Inverse Laplace Transform of 1 / s a + 1 0 e t x a by contour integration
My try:
We get Laplace transform of
g ( t ) = t a
is:
g ^ ( t ) = 1 / s a + 1 0 e t x a
then I was stuck at question 2) which asks me to evaluate the inverse laplace transform of g ^ ( p ) which is
1 / 2 π i 0 e p t g ^ ( p ) d p
I know the answer should be t a as the inverse transform comes back to itself, but I cannot figure out how to make the contour integration. I tried to apply Cauchy's residue theorem to eliminate the 1 / 2 π i but was stuck then.

Answer & Explanation

barene55d

barene55d

Beginner2022-11-17Added 23 answers

g ( t ) t a g ^ ( s ) = 0 t a e s t d t = Γ ( a + 1 ) s a + 1 where Γ ( z ) is the Gamma Function. Also, g ( t ) = γ i γ + i Γ ( a + 1 ) s a + 1 e s t d s 2 π i , γ > 0
g ( t ) = Γ ( a + 1 ) γ i γ + i e s t s a + 1 d s 2 π i = Γ ( a + 1 ) × [ 0 ( s ) a 1 e ( a + 1 ) π i e s t d s 2 π i 0 ( s ) a 1 e ( a + 1 ) π i e s t d s 2 π i ] = Γ ( a + 1 ) [ e π a i 0 s a 1 e s t d s 2 π i e π a i 0 s a 1 e s t d s 2 π i ] = 1 π Γ ( a + 1 ) e π a i e π a i 2 i 0 s a 1 e s t d s = Γ ( a + 1 ) π sin ( π a ) t a   0 s a 1 e s d s =   Γ ( a )
g ( t ) = Γ ( 1 + a ) Γ ( a ) sin ( π a ) π t a
With Euler Reflection Formula , Γ ( 1 + a ) Γ ( a ) sin ( π a ) π = 1 such that
g ( t ) = t a

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