# The CDF from the characteristic function

• I

## Main Question or Discussion Point

This thread will be a collection of multiple questions I asked before over different forums. I will start from the beginning, and I hope someone will follow the steps with me, because I did it before alone, and I ended with a numerical integration that is not finite, which doesn't make sense, since the result should be between 0 and 1.

My main problem is to find the cumulative distribution function (CDF) of the following random variable (RV)

$$Y=\sum_{k=1}^K\underbrace{\frac{a_k}{b_k}}_{X_k}$$

where $\{a_k, b_k\}$ are independent and identically distributed (i.i.d.) exponential RVs with parameter 1.

At first, I wanted to use the moment generating function (MGF), but some pointed out that the RVs $X_k$ have undefined means, and the MGF doesn't work in this case. They suggested the characteristic function (CF), because the CF always exists even if the means don't.

My fist step was then to find the CF of the RV $Y$ as follows

$$\psi_Y(t)=\text{E}_Y\{e^{jtx}\}=\text{E}_{X_1,\,X_2,\ldots,\,X_K}\{e^{jt\sum_{k=1}^KX_k}\}=\prod_{k=1}^K\text{E}_{X_k}\{e^{jtx_k}\}=\prod_{k=1}^K\psi_{X_k}(t)$$

So, the next step would be to find $\psi_{X_k}(t)$, which is given by

$$\psi_{X_k}(t) = \text{E}_{X_k}\{e^{jtx_k}\} = \int_0^{\infty}e^{jtx_k}\,f_{X_k}(x_k)\,dx_k$$

where $f_{X_k}(x_k)$ is the probability distribution function (PDF) of the RV $X_k$. It can be shown that $f_{X_k}(x_k)=1/(1+x_k)^2$, and thus

$$\psi_{X_k}(t) = \int_0^{\infty}\frac{e^{jtx_k}}{(1+x_k)^2}\,dx_k$$

My first question is how to evaluate this integral?

My approach: by integration by parts, this results in

$$\psi_{X_k}(t) = 1+jt\int_0^{\infty}\frac{e^{jtx_k}}{1+x_k}\,dx_k=1+jt\int_0^{\infty}\frac{\cos(tx_k)}{1+x_k}\,dx_k-t\int_0^{\infty}\frac{\sin(tx_k)}{1+x_k}\,dx_k$$

which can be written in terms of cosine and sine integrals as

$$\psi_{X_k}(t) = 1+jt\left[-\sin(t)\text{si}(t)-\cos(t)\text{ci}(t)\right]-t\left[\sin(t)\text{ci}(t)-\cos(t)\text{si}(t)\right]$$

where $\text{si}(t)=-\int_t^{\infty}\sin(z)/z\,dz$ and $\text{ci}(t)=-\int_t^{\infty}\cos(z)/z\,dz$.

Is what I did so far correct? I appreciate if some comment on this, because apparently I made a mistake in my mathematical analysis that resulted in an indefinite integral, but not sure where my mistake is.

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mathman
What indefinite integrals?

I meant the integral gave undefined results. Anyway, after some mathematical manipulation, I reached to the formula

$$\psi_{X_k}(t)=1+je^{-jt}E_1(-jt)$$

where $E_1(x)=\int_x^{\infty}\frac{e^{-z}}{z}\,dz$ is the exponential integral. Since $\{X_k\}$ are i.i.d. RVs, then the CF of the RV $Y$ is given by

$$\psi_Y(t)=\left[1+je^{-jt}E_1(-jt)\right]^K=\sum_{m=0}^K{K\choose m}j^me^{-mjt}E_1^m(-jt)$$.

Now I want to find the CDF of $Y$ from this CF. I found this theorem (Gil-Pelaez)

$$F_Y(y)=\frac{1}{2}-\frac{1}{\pi}\int_0^{\infty}\frac{\text{Im}\left\{e^{-jty}\psi_Y(t)\right\}}{t}\,dt$$

where Im(.) is the imaginary part of a complex number. However, evaluating the above integral numerically gave results that are not real and between 0 and 1, which is supposed to be. Why? What is wrong in the integral? Some said that dividing by t in the integral, while having the limits start from 0 is the problem, but this is the theorem of inverting the CF!! Did I make any mistake in the derivations?

mathman
See what happens if you used this formula to get the distribution function for one X.. Also try to get the density function. Also I don''t see why the CDF is coming out not real. You should take the imaginary part, as a real function.

See what happens if you used this formula to get the distribution function for one X.. Also try to get the density function. Also I don''t see why the CDF is coming out not real. You should take the imaginary part, as a real function.
When I tried to find the density of one of the Xs from the characteristic function using Mathematica, it gave me DiracDelta[x], which is wrong, because the PDF is $f_X(x)=1/(1+x)^2$. Does it mean that there is something wrong in my derivation? The derivation is shown above, and I don't see where I could have made a mistake!

mathman
I tried to check your result for char. function for one X. I found a discrepancy, specifically the si and ci terms should include the values at x=∞

I tried to check your result for char. function for one X. I found a discrepancy, specifically the si and ci terms should include the values at x=∞
What do you mean? Could you elaborate, please?

mathman
I use Gradshteyn and Ryzhik as a reference. Their definite integral agrees with you, but the indefinite integral requires the extra terms when the limits are applied. I am confused myself. In any case I don't understand why you don't get the original density for one X when you take the inverse transform. If you look up the reference, the indefinite integrals are in section 2.641, wile the definite integrals are in section 3.722.

One further question - how did you get the density function of one X?

.... In any case I don't understand why you don't get the original density for one X when you take the inverse transform. If you look up the reference, the indefinite integrals are in section 2.641, wile the definite integrals are in section 3.722.
...
That is what I don't understand. Why I don't get the same density from the CF after inverse Fourier Transform. What reference are you talking about here?

....
One further question - how did you get the density function of one X?
$$f_X(x)=\frac{d\,F_X(x)}{d\,x}=\frac{d}{d\,x}\text{Pr}[X\leq x]=\frac{d}{d\,x}\text{Pr}[a/b\leq x]=\frac{d}{d\,x}\int_0^{\infty}\underbrace{\text{Pr}[a/\beta\leq x/b=\beta]}_{F_a(x\,\beta)}f_b(\beta)\,d\,\beta$$

If you substitute in the above equation $F_a(x)=1-e^{-x}$, and $f_b(x)=e^{-x}$, and then evaluate the integral, and take the derivative with respect to $x$ you will get $f_X(x)=1/(1+x)^2$.

mathman
Table of Integrals, Series, and Products, I.S. Gradshteyn and I.M. Ryzhik, Seventh Edition

When you did the inverse transform, did you try getting the density function? That should be easier than the CDF.
I.

Table of Integrals, Series, and Products, I.S. Gradshteyn and I.M. Ryzhik, Seventh Edition

....I.
Yes, I used the definite integrals in 3.722.

....
When you did the inverse transform, did you try getting the density function? That should be easier than the CDF.
I.
Yes, when I didn't get correct answer for the CDF, I tried getting the PDF from the CF using inverse Fourier transform in Mathematica. It didn't give me the same density function!!

mathman
I'll have to see more details to try to guess what happened.

I think I forgot o include $t$ in the CF of $X_k$. The CF is

$$\psi_{X_k}(t)=1+jte^{-jt}E_1(-jt)$$

When I find the inverse Fourier transform of this in Mathematica as

Code:
PsiY = 1 + I t Exp[-I t] ExpIntegralE[1, -I t];
InverseFourierTransform[PsiY, t, w]
it gives me the following result

Code:
Sqrt[2 \[Pi]] DiracDelta[w]
which is not what I expect!!!

mathman
Have you tried using the previous equation (involving cos, sin, ci, si)? I have not used Mathematica, so I don't know what is happening there.
The Mathematica expression is for a jump distribution at 0 (after constant fixing).

Have you tried using the previous equation (involving cos, sin, ci, si)? I have not used Mathematica, so I don't know what is happening there.
The Mathematica expression is for a jump distribution at 0 (after constant fixing).
I have tried this, but Mathematica ran for a while (maybe 10 minutes), and nothing was produced. My computer made a weird noise while do the computation!! I would guess the CPU and memory were fully utilized for the calculation.

mathman
Based on what you are getting from Mathematica, my guess is that it is not working properly for this char. function. The Fourier transform of a delta function is a constant. Possibly Mathematica cannot handle a function of the form 1+ something. It looks to me that after the original integration by parts, you have an expression where the parts themselves are not invertable and Mathematica can't handle it.

I suggest you post your question on the following Forum. There are many more mathematicians than here who might have an answer..
https://math.stackexchange.com/

I did post it on stack exchange at the same time when I posted this question here, but my question got no single response there!!

mathman
I am at a loss as to what to suggest. In one of the notes, you tried to get the CDF from the char. function. Have you tried it for one X?

mathman
The expression for the Gil-Palaez form for the distribution function is real. For one X it looks like this:
$$F_X(x)=\frac{1}{2}-\frac{1}{\pi}\int_0^{\infty}(\frac{sin(tx)}{t}-si(t)sin(xt+t)-ci(t)cos(xt+t))dt$$.
See what you can do with it.

• EngWiPy
mathman
Simplification: $$\frac{1}{\pi}\int_0^\infty \frac{sin(tx)}{t}dt=\frac{1}{2}$$for all x > 0, so $$F_X(x)$$ is remaining term.

• EngWiPy
The other terms seem to evaluate to simple form values depending on the arguments. For example:

$$\int_0^{\infty}\text{si}(qx)\sin(px)\,dx=\left\{\begin{array}{ll}-\frac{\pi}{2p}&p^2>q^2\\-\frac{\pi}{4p}&p^2=q^2\\0&p^2<q^2\end{array}\right.$$

Which is nice. I ill check this more thoroughly later.

Last edited:
mathman
Since x> 0, p(=1+x) > q(=1). Both integrals are the same, so the net result is $$\frac{-1}{1+x}$$. It looks almost like the right answer which is $$\frac{x}{1+x}$$.. This could be explained if I have the wrong sign for term leading to the integral of $$\frac{sin(t)}{t}$$. In that case, the integral becomes what it should be.

I just checked my notes and I believe I do have the wrong sign for that term. I suggest you try to get the integrand for Gil-Palaez.

I will check this, but $x\geq 0$, not $x>0$. I think we need to take into account the two possibilities that $p=q$ or $p>q$, don't we?

The expression for the Gil-Palaez form for the distribution function is real. For one X it looks like this:
$$F_X(x)=\frac{1}{2}-\frac{1}{\pi}\int_0^{\infty}(\frac{sin(tx)}{t}-si(t)sin(xt+t)-ci(t)cos(xt+t))dt$$.
See what you can do with it.
I checked this, and it is supposed to be like this

$$F_X(x)=\frac{1}{2}+\frac{1}{\pi}\int_0^{\infty}\left(\frac{\sin(tx)}{t}+\text{si}(t)\sin(xt+t)+\text{ci}(t)\cos(xt+t)\right)dt$$

which results in

$$F_X(x)=1-\frac{1}{x+1}$$

for $x>0$, which is the exact CDF of $X$. But if $x=0$, it yields

$$F_X(x)=1-\frac{1}{2(x+1)}$$

This is good, though. Now how can I use this insight for the summation of $X$s, instead of one $X$?

mathman
Your expression for $$F_X(x),\ x=0$$ is wrong. $$\int_0^\infty \frac{sin(tx)}{t}dt=0$$ at that point. The only insight I can give is the expansion to get the imaginary part you need will have the $$\frac{sin(tx)}{t}$$ as the first term while the rest will not be divided by t.