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The representation of functions (or objects which are in some generalize the notion of function) as constant linear combinations of sines and cosines at integer multiples of a given frequency, as Fourier transforms or as Fourier integrals.

4 votes
3 answers
506 views

Fourier transform in $L^1$?

Let $f \in L^1 \cap L^2$. Are there any natural conditions on $f$ that ensure that the Fourier transform $\hat f$ is in $L^1?$ I don't want to have anything as restrictive as Schwartz. I am rather loo …
António Borges Santos's user avatar
2 votes
1 answer
205 views

Asymptotics for oscillatory integral

Consider the following integral for $f \in C_c^{\infty}(\mathbb R^n)$, $x_0$ fixed (possibly zero), and $n \ge 3$ $$F(\lambda) = \int_{\mathbb R^n} e^{i\lambda \vert x-x_0 \vert^2} \frac{f(x)}{\vert x …
António Borges Santos's user avatar
1 vote
0 answers
98 views

Periodicity in one Fourier variable

Let $f:[0,1]\times [0,1] \to \mathbb C$ be a double periodic function (periodic in both variables) that depends real-analytically on its argument. We can thus write $f$ as $$ f(x) = \sum_{n \in \mathb …
António Borges Santos's user avatar
0 votes
1 answer
76 views

Fourier transform of exponential over torus

I found the following formula for the Fourier transform on a flat 2-torus, but I don't quite know how to derive it. We have a variable $q=(q_x,q_y) \in [0,2\pi)^2$ and by considering it in polar coord …
António Borges Santos's user avatar