# Problem with the Fourier transform of |sin(x)|

The code FourierTransform[Abs[Sin[t]], t, w] gives me a timeout like "This computation has exceeded the time limit for your plan."

What indeed is the FT of $$|\sin(x)|$$ ? Why can't Mathematica compute it ?

• The Fourier transform of $|\sin t|$ does not exist in traditional math because the integral $\int_{\mathbb{R}} |\sin t|\,dt$ diverges (see encyclopediaofmath.org/index.php/Fourier_transform for info). Feb 13 '20 at 17:03
• @user64494 $|\sin t|$ is tempered so its Fourier transform is perfectly well-defined. This is explained in your own link ("In classical analysis such a generalization has been constructed for locally integrable functions..."). Please stop making wrong statements about distributions, or learn the basics first. Feb 14 '20 at 0:04
• Pulling an actual book from my shelves, I find that Bracewell has the transform of Abs[Cos[t]] in his "pictorial dictionary". Adjusting for his eccentric scaling, it's the same as the series in the answers below, but with signs of terms alternating to get the phasing right. Feb 14 '20 at 2:40

Abs isn't analytic, and using it here seems to lead FourierTransform to get lost in the complex plane. Use RealAbs:

FourierTransform[RealAbs[Sin[t]], t, w]


yielding:

Since the function is periodic, its transform is a train of delta functions. Something like FourierSeries might be more illuminating here. Maybe something like:

ComplexExpand[FourierSeries[RealAbs[Sin[t]], t, 10]]


yielding:

is what you want.

• thanks for the answer! could I get further insight by perhaps getting the magnitude of this complex result? Abs[%] does not seem to work because of the infinite sum. Feb 13 '20 at 15:29
• The result of FourierTransform[RealAbs[Sin[t]], t, w] is [CASE:4370384] reported by me on 08.01.20 This result is not a closed-form expression, but an analytic expression (see en.wikipedia.org/wiki/Closed-form_expression ). It is difficult to work with it. I was answered that the Mathematica devolopers were informed about that. Feb 13 '20 at 16:32
• @user64494 : what closed-form do you expect for an infinite train of delta functions? Feb 13 '20 at 17:37
• @user64494 : but it gives something that appears to be a correct answer. What's wrong with that? Feb 13 '20 at 20:42
• @user64494: it exists in the pragmatic sense that we use in applied physics, and my FourierSeries result along with @mikado's DiracComb result are perhaps better representations. Feb 13 '20 at 21:50

You can obtain a simple expression using the convolution theorem. A single cycle of the waveform is given by

sig = Sin[t] UnitBox[t/π - 1/2]


This has a relatively simple Fourier Transform

spec = FullSimplify[FourierTransform[sig, t, ω]]
(* -((1 + E^(I π ω))/(Sqrt[2 π] (-1 + ω^2))) *)


We can convert our single cycle into a periodic signal by convolution with a Dirac Comb. This transforms to another Dirac comb

combspec = FourierTransform[DiracComb[t/π], t, ω]
(* Sqrt[π/2] DiracComb[ω/2] *)


By the convolution theorem (this probably needs multiply by a something like 2π), the Fourier transform of periodic signal is given by

Assuming[ω/2 ∈ Integers, FullSimplify[combspec*spec]]
(* DiracComb[ω/2]/(1 - ω^2) *)


Note that we can simplify the factor in front of the comb, as its value only needs to be correct for cases where the comb is non-zero.

• However, the actions of FourierTransform and InverseFourierTransform on DiracDelta related functions are not reliable and correct in many cases (see that recent discussion mathematica.stackexchange.com/questions/212352/… ) . Feb 13 '20 at 21:43
• With this excellent answer, we now have three different approaches yielding equivalent results. Feb 13 '20 at 22:46