What f(t) satisfies the inverse Laplace transform ccL^(-1){(p′(s))/(p(s))}=f(t), where the polynomial p is given.

Paula Cameron

Paula Cameron

Answered question

2022-11-06

What f(t) satisfies the inverse Laplace transform L 1 { p ( s ) p ( s ) } = f ( t ) , where the polynomial p is given.

Answer & Explanation

Antwan Wiley

Antwan Wiley

Beginner2022-11-07Added 13 answers

Let n = deg ( p ) be the degree of the polynomial p. Then by partial fractions we may write
p ( s ) p ( s ) = k = 1 n A k s σ k ,
for some A k , where p ( σ k ) = 0 for all 1 k n. But since 1 s a = L { e a t } , then
k = 1 n A k s σ k = k = 1 n A k L { e σ k t } = L { k = 1 n A k e σ k t } .
Hence,
L 1 { p ( s ) p ( s ) } = k = 1 n A k e σ k t .
It may be that the right hand side has closed form due to the exponential forms of the sine and cosine functions, depending on our choice of p.
Note - this generalises the original question since p′/p≡q/p, where deg(q)However, in the original case for p / p, it appears that A k = 1 for all k, in which case there will indeed be closed fom in terms of sines, cosines, and their hyperbolic equivalents.

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