# Parseval identity in one side Fourier Transform

Is there any posibility to apply Parseval identity(energy in time domain is equal to freq. domain) on One/Single-side Fourier transform?

In other word for "standard" FT the test Mathematica can be

nn = 250; T = 20.;
t = Range[0, T, T/nn];
f = Exp[-(t - 5)^2]*Sin[t - 5];
ts = t[] - t[];
ws = 2 π/ts/(nn);
F = RotateLeft[Abs[Fourier[f]], Floor[Length[f]/2]];
w = ws*Range[-Floor[nn/2] - 1, Floor[nn/2]];
q1 = Total[(w[] - w[])*Abs[F]^2];
q2 = Total[ts*Abs[f]^2];
Fp = F*Sqrt[(q2/q1)];
aries = FourierTransform[
Exp[-(τ - 5)^2]*Sin[τ - 5], τ, ω];
ListLinePlot[Partition[Riffle[t, f], 2], PlotRange -> All]
Show[ListPlot[Partition[Riffle[w, Fp], 2]],
Plot[Abs[aries], {ω, -7, 7}, PlotStyle -> Red],
PlotRange -> All]


One side FT is used for example damped oscilator where t=>0 where the solution can be Exp[-1 t]*Sin[2. t] . The script is

dt = 0.05;
t = Range[0, 10, dt];
fun = Exp[-1 t]*Sin[2. t];
vys = Assuming[ω > 0,
Integrate[
Exp[-tt]*Sin[2 tt]*Exp[I ω tt], {tt, 0, ∞}]];
r = RotateLeft[Im[Fourier[fun]], Floor[Length[Im[Fourier[fun]]]/2]];
r1 = RotateLeft[Fourier[fun], Floor[Length[Im[Fourier[fun]]]/2]];
an = Table[{ω, Im[vys]}, {ω, 0, 10, 0.1}];
dw = 2 π/dt/Length[t];
w = dw*Range[-Floor[Length[t]/2]+1, Floor[Length[t]/2]];
a = Total[t*fun^2]
b = Chop[Total[dw*r1*Conjugate[r1]]];
Show[ListPlot[Partition[Riffle[w, r], 2], PlotRange -> All],
ListLinePlot[Partition[Riffle[an[[All, 1]], an[[All, 2]]], 2],
PlotRange -> All], PlotRange -> All]


I have numerical data(in this case generated with function fun = Exp[-1 t]*Sin[2. t];). I like to evaluate integral((Integrate[Exp[-tt]*Sin[2 tt]*Exp[I ω tt], {tt, 0, ∞}]])) on this data. I am wondering if is possible to use fft(Fourier command) to evaluate this integral with numerical data. Note, there is analytical and numerical solution for comparision(This is test a script, in the final, there will be only numerical data). The graph shows numerical and analytical solution, the position/freq match is OK, but the amplitude don’t fit.

• I am having a difficult time figuring out what is being done here. Does it help to use a "full" FT with a cut-off function? What I have in mind: vys2 = FourierTransform[Exp[-tt]*Sin[2 tt]*HeavisideTheta[tt], tt, \[Omega]]. – Daniel Lichtblau Nov 11 '15 at 19:24
• Dear Mr. Lichtblau, thank you for your answer. The question was re-editet. Hope that it now much clearer. – Eduard Nov 12 '15 at 14:25

I think you want to pad to the left with zeros. The slight alterations below might put you closer to what you want.

dt = 0.05;
t1 = Range[0, 10, dt];
t = Join[Reverse[-Rest[t1]], t1];
fun1 = Exp[-1 t1]*Sin[2. t1]; (* Use the nonnegative values here *)
fun = PadLeft[fun1, 2*Length[fun1] - 1]; (* Now pad on the negative side with zeroes *)
(* The purpose here is to emulate the fact that for negative values your function under consideration should be zero *)

vys = Assuming[Element[\[Omega], Reals],
Integrate[
HeavisideTheta[tt]*Exp[-tt]*Sin[2 tt]*
Exp[I \[Omega] tt], {tt, -\[Infinity], \[Infinity]}]];
r = RotateLeft[Im[Fourier[fun]], Floor[Length[Im[Fourier[fun]]]/2]];
r1 = RotateLeft[Fourier[fun], Floor[Length[Im[Fourier[fun]]]/2]];

an = Table[{\[Omega], Im[vys]}, {\[Omega], 0, 10, 0.1}];
dw = 2 \[Pi]/dt/Length[t];
w = dw*Range[-Floor[Length[t]/2] + 1, Floor[Length[t]/2]];

a = Total[t*fun^2]
b = Chop[Total[dw*r1*Conjugate[r1]]];

(* Out= 2.80000401905 *)

Show[ListPlot[Partition[Riffle[w, r], 2], PlotRange -> All],
ListLinePlot[Partition[Riffle[an[[All, 1]], an[[All, 2]]], 2],
PlotRange -> All], PlotRange -> All] If this is way off then you really need to make more clear what it is you are trying to do.

I too am not clear what you want. I think you may need to use FourierParameters correctly. I will give you an illustration using your example.

nn = 250; T = 20.;
t = Range[0, T, T/nn];
f = Exp[-(t - 5)^2]*Sin[t - 5];


I will now work out the mean square value of the time values

f.f/Length[f]

(* 0.0122794 *)


Now we take the Fourier transform and use one example of FourierParameters

ft = Fourier[f, FourierParameters -> {-1, -1}];


The values are complex and we work out the sum of their modulus values squared.

ft.Conjugate[ft]


(* 0.0122794 + 0. I*)

This is the same value as above. If you wish to use half of the Fourier spectrum then you will be out by a factor of two. Thus

ft2 = ft[[1 ;; Round[Length[ft]/2]]];
ft2.Conjugate[ft2]
(*0.0061397 + 0. I *)


I have put some details on basics of Fourer here. You may choose alternative values for FourierParameters which may suit your needs better. As illustrated Fourier does obey Parsevals theorem.