# Roots of random variables

Given a univariate random variable $$Z$$ with density $$f_Z$$, and a polynomial $$Q[X_1, \ldots, X_n]$$, we are interested in finding a random variable $$X$$ such that, given independent copies of it $$X_1, \ldots, X_n$$ (where $$\stackrel{\text{D}}{=}$$ stands for the equality in distribution):

$Z \stackrel{\text{D}}{=} Q[X_1, \ldots, X_n].$

This is a different problem from studying the root of a random polynomial, where the coefficients are random but not the variables. This is also different from computing $$Z$$ given $$X$$ (which is the inverse problem).

We only consider the case of multivariate monomials ($$X_1^3, X_1 X_2 X_3, X_1^2 X_2, \ldots$$) and of some common distributions on $$\mathbb{R}$$ (normal, exponential, gamma distributions).

We consider at first the cases $$X_1^k$$ and $$X_1 \ldots X_k$$ before moving to the monomials up to degree five (the number of monomials of degree $$k$$ is given by the partition numbers):

• $$X_1$$,
• $$X_1^2$$, $$X_1 X_2$$,
• $$X_1^3$$, $$X_1^2 X_2$$, $$X_1 X_2 X_3$$,
• $$X_1^4$$, $$X_1^3 X_2$$, $$X_1^2 X_2^2$$, $$X_1^2 X_2 X_3$$, $$X_1 X_2 X_3 X_4$$,
• $$X_1^5$$, $$X_1^4 X_2$$, $$X_1^3 X_2^2$$, $$X_1^3 X_2 X_3$$, $$X_1^2 X_2 X_3 X_4$$, $$X_1^2 X_2^2 X_3$$, $$X_1 X_2 X_3 X_4 X_5.$$

## Recall of some distributions

$$\text{Gamma}(k, \theta)$$, with shape $$k > 0$$ and scale $$\theta > 0$$ is given by the density (for $$x > 0$$):

$\frac{1}{\Gamma{\left( k \right)} \theta^k} x^{k-1} e^{-x/ \theta}.$

$$\text{GGamma}(k, \theta, p)$$, with shape $$k > 0$$, scale $$\theta > 0$$, power $$p > 0$$ is given by the density (for $$x > 0$$):

$\frac{p}{\Gamma{\left( k/p \right)} \theta^k} x^{k-1} e^{-\left( x/\theta \right)^p}.$

## Root of $$X^k = Z$$

### Case of positive variables

For $$Z$$ distributed on $$\mathbb{R}^{+}$$, and $$X$$ searched as nonnegative too, a change of variable gives the following density function, for $$x \geq 0$$:

$f_X(x) = k x^{k-1} f_Z(x^k).$

A way of generating $$X$$ from $$Z$$ is given by:

$X = Z^{1/k}.$

### Other cases

#### Case $$Z \geq 0$$ and $$X \in \mathbb{R}$$

For $$k = 2$$, if $$Z$$ is nonnegative but $$X$$ is possibly real, then $$X = -\sqrt{Z}$$ also works. But $$X = (-1)^{\mathbf{1}_{Z \leq 1}} \sqrt{Z}$$ also.

Example of three random variables $$X$$ such that $$X^2$$ follows $$\text{Exp}(1)$$.
png("1_three_variables_such_that_square_is_exp.png", 827, 400, pointsize=24)
N = 1e7
lambda = 1
k = 2
Z = rexp(N, lambda)

par(mfrow=c(1,3))

## Positive square-root
X = Z^(1/k)
hist(X, probability = TRUE, breaks = 300,
main = "Positive X")
x = seq(from = -10, to = 10, length.out = 1000)
lines(x, k * x^(k-1) * dexp(x^k, lambda) * (x > 0), col = "red")

## Negative square-root (only for k = 2)
X = -Z^(1/k)
hist(X, probability = TRUE, breaks = 300,
main = "Negative X")
x = seq(from = -10, to = 10, length.out = 1000)
lines(x, k * abs(x)^(k-1) * dexp(x^k, lambda) * (x < 0), col = "red")

## Another alternative square-root (only for k = 2)
X = ifelse(Z <= 1, Z^(1/k), -Z^(1/k))
hist(X, probability = TRUE, breaks = 300,
main = "Alternative X")
x = seq(from = -10, to = 10, length.out = 1000)
lines(x, k * abs(x)^(k-1) * dexp(x^k, lambda) * (x < -1 | (x < 1 & x > 0)), col = "red")

# # Check that Z is here after taking the power value
# hist(X^k, probability = TRUE, breaks = 300)
# lines(x, dexp(x, lambda), col = "red")
dev.off()


#### Case $$Z \in \mathbb{R}$$

If $$Z$$ has a positive probability to be negative, the variable $$X$$ may need to live on $$\mathbb{C}$$ to exist. For example for $$k=2$$ and $$Z$$ a random variable following the normal distribution, defining $$X := \sqrt{\mid Z \mid}$$ if $$\text{sign}(Z) \geq 0$$ and $$X := i \sqrt{\mid Z \mid}$$ otherwise will work.

## Root $$\sqrt[k]{Z}$$ given by $$X_1 \ldots X_k = Z$$

### Case normal

We provide a way of generating $$X_1$$ as follows. We define independent variables $$\varepsilon$$ following $$\frac{1}{2} \mathbf{1}_{\lbrace -1, 1 \rbrace}$$, and $$G_{1/k, 0}, G_{1/k, 1}, \ldots$$ each following $$\text{Gamma}(1/k, 1)$$.

$X_1 := \varepsilon \exp \left\lbrace \frac{\log 2}{2k} - G_{1/k, 0} - \sum_{j=1}^{+\infty} \left[ \frac{G_{1/k, j}}{2j+1} - \frac{1}{2k} \log \left( 1 + \frac{1}{j} \right) \right] \right\rbrace.$

Other $$X_i$$ are independent copies of $$X_1$$, and we have $$X_1 \ldots X_k$$ following the normal distribution.

This is exactly the results obtained by Iosif Pinelis in: The exp-normal distribution is infinitely divisible.

Proof.

Let $$Z$$ be a standard normal random variable. The distribution $$U := \log |Z|$$ is referred to as the exp-normal distribution, and its characteristic function is, for $$t \in \mathbb{R}$$:

\begin{align*} \mathbb{E} e^{it\log |Z|} =& \int_{-\infty}^{+\infty} e^{it \log |z|} \frac{1}{\sqrt{2\pi}} e^{-z^2/2} dz \\ \text{(Symm. in z)}=& 2 \int_{0}^{+\infty} e^{it \log z} \frac{1}{\sqrt{2\pi}} e^{-z^2/2} dz \\ =& \frac{2}{\sqrt{2\pi}} \int_{0}^{+\infty} e^{it \log z -z^2/2} dz \\ =& \frac{2}{\sqrt{2\pi}} \left[ 2^{i(i+t)/2} \Gamma \left(\frac{1+it}{2} \right) \right] \\ =& \frac{2^{1/2}}{\sqrt{\pi}} 2^{\frac{it -1}{2}} \Gamma \left(\frac{1+it}{2} \right) \\ =& \frac{2^{it/2}}{\sqrt{\pi}} \Gamma \left(\frac{1+it}{2} \right) \\ =& 2^{it/2} \frac{\Gamma \left(\frac{1+it}{2} \right)}{\Gamma \left( \frac{1}{2} \right)} \\ =& \exp \left( it \frac{\log 2}{2} \right) \Gamma \left(\frac{1+it}{2} \right) \frac{1}{\Gamma \left( \frac{1}{2} \right)} \end{align*}

We use the formula (valid for $$z \in \mathbb{C} \setminus \left\{0, -1, -2, \ldots \right\}$$): $$\Gamma(z) = \frac{1}{z} \prod_{j=1}^{+\infty} \frac{\left( 1 + \frac{1}{j} \right)^z}{1 + \frac{z}{j}}$$

to get:

\begin{align*} \mathbb{E} e^{it\log |Z|} =& \exp \left( it \frac{\log 2}{2} \right) \left[ \frac{2}{1+it} \prod_{j=1}^{+\infty} \frac{\left( 1 + \frac{1}{j} \right)^{\frac{1+it}{2}}}{1 + \frac{1+it}{2j}} \right] \frac{1}{2 \prod_{j=1}^{+\infty} \frac{\left( 1 + \frac{1}{j} \right)^{1/2}}{1 + \frac{1}{2j}}} \\ =& \exp \left( it \frac{\log 2}{2} \right) \left[ \frac{2}{1+it} \prod_{j=1}^{+\infty} \frac{\left( 1 + \frac{1}{j} \right)^{\frac{1+it}{2}}}{1 + \frac{1+it}{2j}} \right] \frac{1}{2} \prod_{j=1}^{+\infty} \frac{1 + \frac{1}{2j}}{\left( 1 + \frac{1}{j} \right)^{1/2}} \\ =& \exp \left( it \frac{\log 2}{2} \right) \frac{1}{1+it} \left[ \prod_{j=1}^{+\infty} \frac{\left( 1 + \frac{1}{j} \right)^{\frac{1+it}{2}}}{1 + \frac{1+it}{2j}} \right] \prod_{j=1}^{+\infty} \left[ \left(1 + \frac{1}{2j} \right) \left( 1 + \frac{1}{j} \right)^{-1/2} \right] \\ =& \exp \left( it \frac{\log 2}{2} \right) \frac{1}{1+it} \left[ \prod_{j=1}^{+\infty} \left( 1 + \frac{1}{j} \right)^{\frac{it}{2}} \right] \prod_{j=1}^{+\infty} \frac{1 + \frac{1}{2j}}{1 + \frac{1+it}{2j}} \\ =& \exp \left( it \frac{\log 2}{2} \right) \frac{1}{1+it} \left[ \prod_{j=1}^{+\infty} \left( 1 + \frac{1}{j} \right)^{\frac{it}{2}} \right] \prod_{j=1}^{+\infty} \frac{1}{1 + \frac{it}{2j+1}} \\ =& \exp \left( it \frac{\log 2}{2} \right) \frac{1}{1+it} \prod_{j=1}^{+\infty} \exp \left[ \frac{it}{2} \log \left( 1 + \frac{1}{j} \right) \right] \prod_{j=1}^{+\infty} \frac{1}{1 + \frac{it}{2j+1}} \\ =& \exp \left( it \frac{\log 2}{2} \right) \frac{1}{1+it} \prod_{j=1}^{+\infty} \frac{\exp \left[ \frac{it}{2} \log \left( 1 + \frac{1}{j} \right) \right]}{1 + \frac{it}{2j+1}}. \end{align*}

In addition, the characteristic function of an exponential variable $$X$$ with mean $$a > 0$$ (so with parameter $$1/a$$) is:

$\mathbb{E} e^{itX} = \frac{1}{a} \int_0^{+\infty} e^{itx} e^{-x/a} dx = \frac{1}{a} \frac{1}{\frac{1}{a}-it} = \frac{1}{1-ita}.$

With $$a=1$$, the characteristic function of the variable $$-X$$ is:

$\mathbb{E} e^{-itX} = \frac{1}{1+ita} = \frac{1}{1+it}.$

and for the variable $$-X/(2j+1)$$ (still with $$a=1$$), it is:

$\mathbb{E} e^{-it\frac{X}{2j+1}} = \frac{1}{1+\frac{ita}{2j+1}} = \frac{1}{1+\frac{it}{2j+1}}.$

In addition, the characteristic function of the constant $$\frac{\log 2}{2}$$ is $$\exp \left( it \frac{\log 2}{2} \right)$$; and for the constant $$\frac{\log \left(1 + \frac{1}{j} \right)}{2}$$ it is $$\exp \left( it \frac{\log \left( 1 + \frac{1}{j} \right)}{2} \right)$$.

We have a product of characteristic distribution, so given $$E_0, E_1, \ldots$$ independent exponential random variable with parameter $$1$$, so have this equality in distribution:

$\log |Z| = \frac{\log 2}{2} - E_0 - \sum_{j=1}^{\infty} \left[ \frac{E_j}{2j+1} - \frac{1}{2} \log \left( 1 + \frac{1}{j} \right) \right].$

Since the sum of $$k$$ independent $$\text{Gamma}(1/k, 1)$$ distributed variables follows an exponential distribution $$\text{Exp}(1)$$, the result follows.

### Case generalized gamma

TODO

Proof.

Let $$Z$$ be a generalized gamma random variable with shape $$k > 0$$, scale $$\theta > 0$$, and power $$p > 0$$, that is for $$x > 0$$ the density function is:

$f(x) = \frac{p}{\Gamma{\left( k/p \right)} \theta^k} x^{k-1} e^{-\left( x/\theta \right)^p}.$

We have:

$\Gamma \left( \frac{it + k}{p} \right) = \frac{p}{it + k} \prod_{j=1}^{+\infty} \frac{\left( 1 + \frac{1}{j} \right)^{\frac{it + k}{p}}}{1 + \frac{it + k}{jp}}$ $\Gamma \left( \frac{k}{p} \right) = \frac{p}{k} \prod_{j=1}^{+\infty} \frac{\left( 1 + \frac{1}{j} \right)^{\frac{k}{p}}}{1 + \frac{k}{jp}}$ $\frac{\Gamma \left( \frac{it + k}{p} \right)}{\Gamma \left( \frac{k}{p} \right)} = \frac{k}{it + k} \prod_{j=1}^{+\infty} \frac{\left( 1 + \frac{1}{j} \right)^{\frac{it + k}{p}}}{\left( 1 + \frac{1}{j} \right)^{\frac{k}{p}}} \frac{1 + \frac{k}{jp}}{1 + \frac{it + k}{jp}}$

The distribution $$U := \log Z$$ is referred to as the exp-ggamma distribution, and its characteristic function is, for $$t \in \mathbb{R}$$:

\begin{align*} \mathbb{E} e^{it\log Z} =& \int_{0}^{+\infty} e^{it \log z} \frac{p}{\Gamma{\left( k/p \right)} \theta^k} z^{k-1} e^{-\left( z/\theta \right)^p} dz \\ =& \frac{p}{\Gamma{\left( k/p \right)} \theta^k} \int_{0}^{+\infty} e^{it \log z} z^{k-1} e^{-\left( z/\theta \right)^p} dz \\ =& \frac{p}{\Gamma{\left( k/p \right)} \theta^k} \left[ \frac{1}{p} \theta^{k + it} \Gamma \left( \frac{it + k}{p} \right) \right] \\ =& \theta^{it} \frac{\Gamma \left( \frac{it + k}{p} \right)}{\Gamma{\left( \frac{k}{p} \right)}} \\ =& \theta^{it} \left[ \frac{k}{it + k} \prod_{j=1}^{+\infty} \frac{\left( 1 + \frac{1}{j} \right)^{\frac{it + k}{p}}}{\left( 1 + \frac{1}{j} \right)^{\frac{k}{p}}} \frac{1 + \frac{k}{jp}}{1 + \frac{it + k}{jp}} \right] \\ =& \theta^{it} \left[ \frac{1}{1 + \frac{it}{k}} \prod_{j=1}^{+\infty} \left( 1 + \frac{1}{j} \right)^{\frac{it}{p}} \frac{1}{1 + \frac{it}{jp + k}} \right] \\ =& \exp \left( it \log \theta \right) \frac{1}{1 + \frac{it}{k}} \prod_{j=1}^{+\infty} \frac{1}{1 + \frac{it}{jp + k}} \exp \left( it \frac{\log \left( 1 + \frac{1}{j} \right)}{p} \right) \end{align*}

In addition, the characteristic function of an exponential variable $$X$$ with mean $$a > 0$$ (so with parameter $$1/a$$) is:

$\mathbb{E} e^{itX} = \frac{1}{a} \int_0^{+\infty} e^{itx} e^{-x/a} dx = \frac{1}{a} \frac{1}{\frac{1}{a}-it} = \frac{1}{1-ita}.$

With $$a=1$$, the characteristic function of the variable $$-X/k$$ is:

$\mathbb{E} e^{-itX/k} = \frac{1}{1+\frac{it}{k}}.$

and for the variable $$-X/(jp+k)$$ (still with $$a=1$$), it is:

$\mathbb{E} e^{-it\frac{X}{jp+k}} = \frac{1}{1+\frac{ita}{jp+k}} = \frac{1}{1+\frac{it}{jp+k}}.$

In addition, the characteristic function of the constant $$\log \theta$$ is $$\exp \left( it \log \theta \right)$$; and for the constant $$\frac{\log \left( 1 + \frac{1}{j} \right)}{p}$$ it is $$\exp \left( it \frac{\log \left( 1 + \frac{1}{j} \right)}{p} \right)$$.

We have a product of characteristic distribution, so given $$E_0, E_1, \ldots$$ independent exponential random variable with parameter $$1$$, so have this equality in distribution:

$\log Z = \log \theta - \frac{E_0}{k} - \sum_{j=1}^{\infty} \left[ \frac{E_j}{jp+k} - \frac{1}{p} \log \left( 1 + \frac{1}{j} \right) \right].$

Then todo, we can cut it.

## Summary

### For the exponential distribution

#### Square

Variable Way of generating Density
$$Z$$ $$\text{Exp}(\lambda)$$ $$\lambda e^{-\lambda x}$$
$$X^{(2)}$$ $$\sqrt{Z}$$ $$2 \lambda x \exp{-\lambda x^2} \sim \text{Rayleigh} \left( 1 / \sqrt{2 \lambda} \right) = \chi \left( 2, 1 / \sqrt{2 \lambda} \right)$$
$$X^{(1,1)}$$
$$\left( X_1^{(2)} \right)^2$$ $$\text{Exp}(\lambda)$$ $$\lambda e^{-\lambda x}$$
$$X_1^{(2)} X_2^{(2)}$$ todo todo
$$\left( X_1^{(1,1)} \right)^2$$ todo todo
$$X_1^{(1,1)} X_2^{(1,1)}$$ $$\text{Exp}(\lambda)$$ $$\lambda e^{-\lambda x}$$

#### Cubic

$$Z$$ $$X^{(2)}$$ $$X^{(1,1)}$$ $$X^{(2)}$$
$$\text{Exp}(\lambda)$$ $$k x^{k-1} \lambda \exp{-\lambda x^k}$$ nicely
1 2 3
$$Z$$ $$Z^{1/k}$$ After
$$\text{Exp}(\lambda)$$ $$k x^{k-1} \lambda \exp{-\lambda x^k}$$ nicely
1 2 3
$\Gamma(k, \theta) = \chi^2 \left( 2k, \sqrt{\theta / 2} \right)$

y never heard about RNN, you can name to give first.

The present post focuses on understanding computations in each model step by step, without paying attention to train something useful. It is illustrated with Keras codes and divided into five parts:

• TimeDistributed component,
• Simple RNN,
• Simple RNN with two hidden layers,
• LSTM,
• GRU.

This diagram is an illustration of an LSTM cell. Check out part D for details.

Companion source code for this post is available here.

long-dependence series. The main issue is caused by the vanishing gradient problem. This problem is detailed in Section 10.7 of the Deep Learning book.

## Part A: Explanation of the TimeDistributed component

A very simple network. Let’s begin with one-dimensional input and output.

### References

Written on June 11, 2021