Saturday, 16 July 2016

Fourier transform of exponent: Delta pulse or hyperbola?


Why do some tables say that Laplace (or Fourier?) inverse of exponential is a time-shifted delta pulse


\begin{align} \delta (t) &\overset{\mathcal F}{\Longleftrightarrow} 1\\ \delta (t-t_0) &\overset{\mathcal F}{\Longleftrightarrow} e^{-j2\pi f t_0}\\ 1 &\overset{\mathcal F}{\Longleftrightarrow} \delta (f)\\ e^{j2\pi f_0 t} &\overset{\mathcal F}{\Longleftrightarrow}\delta (f-f_0) \end{align}


And table: \begin{array}{c|c} f(t)=\mathcal L^{-1}\left\{\mathcal F(s)\right\}&\mathcal F(s)=\mathcal L\left\{f(t)\right\}\\\hline e^{at}&\displaystyle\frac{1}{s-a} \end{array}


whereas others say that it is a (pole) hyperbola rather than a delta-pulse?



$$ e^{at}\overset{\mathcal F}{\Longleftrightarrow}\frac{1}{s-a} $$




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