Fourier Transform of f'(ax): Discrepancy in Results?

In summary: Y: In summary, the Fourier transform of $f'(ax)$ is $\frac{i \omega}{a^2} F(\frac{\omega}{a})$, as found through substitution and integration by parts. This is different from the result obtained through the scaling theorem and the derivative rule, which has an extra factor of $\frac{1}{a}$. The discrepancy is due to the different approaches used.
  • #1
Poirot1
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What is the Fourier transform of $f'(ax)$, where a>0 is a constant? Firstly, I reasoned that (lets say $F[f]$ is the Fourier transform of f) $F[f('x)]=\frac{1}{a}F[f](\frac{k}{a})$ by scaling theorem, then using the derivative rule we get $F[f'(ax)]=\frac{ik}{a}F[f(x)](\frac{k}{a})$. But when I did it manually (substitution and integration by parts), I got an extra factor of $\frac{1}{a}$. So which is correct and why the discrepency?
 
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  • #2
Poirot said:
What is the Fourier transform of $f'(ax)$, where a>0 is a constant? Firstly, I reasoned that (lets say $F[f]$ is the Fourier transform of f) $F[f('x)]=\frac{1}{a}F[f](\frac{k}{a})$ by scaling theorem, then using the derivative rule we get $F[f'(ax)]=\frac{ik}{a}F[f(x)](\frac{k}{a})$. But when I did it manually (substitution and integration by parts), I got an extra factor of $\frac{1}{a}$. So which is correct and why the discrepency?

Let: \(g(x)=f'(x)\) and \(h(x)=g(a x)\), and \(H(\omega)\), \(G(\omega)\) and \(F(\omega)\) be the FT of \(h(x)\), \(g(x)\) and \(h(x)\) respectively.

then:

\[ H(\omega)=\frac{1}{a}G\left(\frac{\omega}{a}\right) \]

and:

\[ G(\omega) = i \omega F(\omega)\]

Now substitute the second into the first to get:

\[ H(\omega)=\frac{1}{a} \frac{ i \omega}{a}F\left(\frac{\omega}{a}\right) = \; \frac{i \omega}{a^2} F\left( \frac{\omega}{a}\right) \]

CB
 
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Related to Fourier Transform of f'(ax): Discrepancy in Results?

1. What is the Fourier Transform of f'(ax)?

The Fourier Transform of f'(ax) is the mathematical operation that decomposes a function f'(ax) into its constituent frequencies. It is a powerful tool used in signal processing and data analysis.

2. Why are there discrepancies in the results of Fourier Transform of f'(ax)?

Discrepancies in the results of Fourier Transform of f'(ax) can occur due to various factors such as numerical errors, sampling rate, and the chosen window function. These discrepancies can also arise if the function f'(ax) is not well-behaved or if the data is noisy.

3. How can discrepancies in the results of Fourier Transform of f'(ax) be minimized?

To minimize discrepancies in the results of Fourier Transform of f'(ax), it is important to carefully choose the sampling rate and window function. It is also recommended to use multiple window functions and average the results to get a more accurate representation of the function's frequency components.

4. Can the Fourier Transform of f'(ax) be applied to non-periodic functions?

Yes, the Fourier Transform of f'(ax) can be applied to non-periodic functions. However, since the Fourier Transform assumes periodicity, the function should be extended periodically beyond its original domain to get meaningful results.

5. Are there any other transforms that can be used instead of Fourier Transform of f'(ax)?

Yes, there are other transforms such as the Laplace Transform and the Wavelet Transform that can also be used for signal analysis. Each transform has its own advantages and limitations, and the choice depends on the specific application and the type of data being analyzed.

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