# Existence of a random vector such that the differences of its components satisfy some restrictions

Let us fix any three numbers in $$[0,1]$$ and summing up to $$1$$. I denote them by $$p_1, p_2, p_3$$.

Could you help to show that, for every possible vector of reals $$U\equiv (U_0, U_1, U_2)\in \mathbb{R}^3$$, there exists a random vector $$\epsilon\equiv (\epsilon_0, \epsilon_1, \epsilon_2)$$ continuously distributed on $$\mathbb{R}^3$$ such that the following equalities hold: $$\begin{cases} p_1=Pr(\epsilon_1-\epsilon_0\geq U_0-U_1, \epsilon_1-\epsilon_2\geq U_2-U_1)\\ p_2=Pr(\epsilon_2-\epsilon_0\geq U_0-U_2, \epsilon_1-\epsilon_2\leq U_2-U_1)\\ p_3=Pr(\epsilon_1-\epsilon_0\leq U_0-U_1, \epsilon_2-\epsilon_0\leq U_0-U_2) \end{cases}$$

This question is related to a problem of identification in econometrics.

Following the comments below, I first reduce the dimension of my inequalities. In fact, $$\begin{cases} Pr(\epsilon_1-\epsilon_0\geq U_0-U_1, \epsilon_1-\epsilon_2\geq U_2-U_1)=Pr(\eta_1\geq -V_1, \eta_1-\eta_2\geq V_2-V_1)\\\ Pr(\epsilon_2-\epsilon_0\geq U_0-U_2, \epsilon_1-\epsilon_2\leq U_2-U_1)=Pr(\eta_2\geq -V_2, \eta_1-\eta_2\leq V_2-V_1)\\ Pr(\epsilon_1-\epsilon_0\leq U_0-U_1, \epsilon_2-\epsilon_0\leq U_0-U_2)=Pr(\eta_1\leq -V_1, \eta_2\leq -V_2) \end{cases}$$

where $$\eta_1\equiv \epsilon_1-\epsilon_0\\ \eta_2\equiv \epsilon_2-\epsilon_0\\ V_1\equiv U_1-U_0\\ V_2\equiv U_2-U_0\\$$

Consider the regions \begin{aligned} &\mathcal{R}_{1,U}\equiv \{(\eta_1,\eta_2)\in \mathbb{R}^2: \eta_1\geq -V_{1}, \eta_1-\eta_2\geq V_{2}-V_{1}\}\\ & \mathcal{R}_{2,U}\equiv \{(\eta_1,\eta_2)\in \mathbb{R}^2: \eta_2\geq -V_{2}, \eta_1-\eta_2\leq V_{2}-V_{1}\}\\ & \mathcal{R}_{3,U}\equiv \{(\eta_1,\eta_2)\in \mathbb{R}^2: \eta_1\leq -V_1, \eta_2\leq -V_2\}\\ \end{aligned} These regions are non-empty and non-overlapping (except for the edges which, however, have zero probability measure). Further, they have a common vertex with coordinates $$(-V_{1},-V_{2})$$.

I now construct a continuous distribution for $$(\eta_1, \eta_2)$$ such that $$$$\label{eta_system} \begin{cases} p_1=Pr(\eta_1\geq -V_1, \eta_1-\eta_2\geq V_2-V_1)\\\ p_2=Pr(\eta_2\geq -V_2, \eta_1-\eta_2\leq V_2-V_1)\\ p_3=Pr(\eta_1\leq -V_1, \eta_2\leq -V_2) \end{cases}$$$$ Consider a bivariate normal distribution, $$\mathcal{N}_2(\mu, \Sigma_{\kappa_1,\kappa_2})$$ with mean $$\mu\equiv (-V_1,-V_2)$$ and variance-covariance matrix $$\Sigma_{\tau_1,\tau_2}\equiv \begin{pmatrix} 5 & \tau_1\\ \tau_1 & \tau_2 \end{pmatrix}$$ We can show that there exists values for $$(\tau_1,\tau_2)$$ such that system above is satisfied for $$\eta\sim \mathcal{N}_2(\mu, \Sigma_{\tau_1,\tau_2})$$ [HOW?].

Let $$\epsilon_0\sim \mathcal{N}(0,1)$$. Let $$\epsilon_1\equiv \eta_1+\epsilon_0$$ and $$\epsilon_2\equiv \eta_2+\epsilon_0$$. These $$\epsilon$$ satisfy my original system

• You can reduce the dimension by one upon observing that all the information given in your question concerns only the 2D vectors $(U_2-U_1,U_1-U_0)$ and $(\epsilon_2-\epsilon_1,\epsilon_1-\epsilon_0).$ That permits you to draw plots of your inequalities. Why don't you do that and see whether the solution then becomes apparent? – whuber Sep 19 '20 at 16:26
• Well done. Given that bivariate Normal distributions have five parameters and you impose only three numerical conditions, generically you should be able to find a two-dimensional family of Normal distributions that satisfies them. – whuber Sep 20 '20 at 14:55
• Independently give $\epsilon_0$ any distribution you like. – whuber Sep 20 '20 at 15:35
• To the latter: yes, because you can define $\epsilon_1=\epsilon_0+\eta_1$ and $\epsilon_2=\epsilon_0+\eta_2.$ The former requires a construction. You can obtain an explicit one by setting the mean at the common vertex of the three regions, fixing one of the standard deviations, and adjusting the other standard deviation and the correlation coefficient to satisfy the two independent probability conditions. – whuber Sep 20 '20 at 17:02
• You can actually solve it explicitly. – whuber Sep 21 '20 at 13:01

Your result is true -- and you have tremendous freedom in selecting the underlying distribution of $$(\eta_1,\eta_2).$$

Your regions consist of three (infinite) angular sectors located at $$(-V_1,-V_2).$$ Let $$F$$ be any continuous distribution in the plane. According to a result which I state and prove rigorously below, it is possible to shift and (uniformly) scale $$F$$ to assign any specified probabilities $$p_i$$ to their respective regions $$\mathcal{R}_i,$$ at least with arbitrarily small error (and with no error when all the $$p_i$$ are positive), assuming only that the $$p_i$$ are consistent with the axioms of probability: they must be non-negative and sum to unity.

Given $$F$$ and $$(p_1,p_2,p_3),$$ assume (with no loss of generality) that $$F$$ has been shifted and scaled as just described. Let $$(\epsilon_0,\eta_1,\eta_2)$$ be any 3D random vector for which $$(\eta_1,\eta_2)$$ has distribution $$F.$$ By construction, the random vector $$(\epsilon_0, \epsilon_0+\eta_1, \epsilon_0+\eta_2)$$ satisfies all your requirements.

This figure illustrates the ideas using the notation introduced below.

The boundaries of the three regions of the question are shown at left. For this construction I have chosen $$F$$ to be the standard Normal distribution (in $$\mathbb{R}^2$$) and kept $$\sigma$$ fixed at $$1,$$ thereby varying only its origin $$\mu.$$ When $$\mu$$ follows one of the colored circles at the left, the corresponding probabilities assigned to the three regions are plotted by the same colored curve at the right, which shows a map of the 2-simplex $$S_2$$ representing all possible such probability triples. It is evident that any probability triple $$(p_1,p_2,p_3)$$ you might care to assign to the three regions can be realized in this way.

Note that in the illustration we can take $$\epsilon_0$$ to be standard Normal and independent of the other variables, whence *every one of these solutions is the standard Normal distribution (in $$\mathbb{R}^3$$) shifted by $$(0,\mu_1,\mu_2)$$ for a suitable vector $$\mu=(mu_1,mu_2)$$ determined uniquely by $$(p_1,p_2,p_3).$$

I recommend finding $$\mu$$ numerically. I used the pmvnorm in the mvtnorm package for R to compute the sector probabilities -- the rest is then easy (just minimize the squared Euclidean norm $$|\Phi(\mu,1)-(p_1,p_2,p_3)|^2:$$ see the function f in the code at the end.) This required some work because this package only computes probabilities of rectangles: you first have to transform an angular sector into a rectangle (either a quadrant, half plane, or complement of a quadrant) using an area-preserving transformation and then apply pmvnorm. The details are given by the function psector at the end of this post.

The regions $$\mathcal{R}_i$$ don't need to be infinite angular sectors in the plane. In order to carry out the following construction, we need only assume the plane has been partitioned into three disjoint measurable regions $$\mathcal{R}_i,$$ each with nonempty interior, in such a way that between any two regions indexed by $$i$$ and $$j$$ there is a continuous path $$\gamma_{ij}$$ lying wholly in the interior of $$\mathcal{R}_i\cup \mathcal{R}_j$$ connecting some point in the interior of $$\mathcal{R}_i$$ to some point in the interior of $$\mathcal{R}_j.$$ Let's call this a "good" partition. (There exist partitions of the plane that do not have this property!) For future reference, designate three "base points" $$x_i\in\mathcal{R}_i$$ lying in the interiors.

Let $$X$$ be any continuous random variable in the plane associated with a distribution function $$F(\mathcal A) = \Pr(X\in\mathcal A)$$ to any measurable plane set $$\mathcal A.$$ To find a solution, we are going to vary $$F$$ by (a) shifting it and (b) uniformly rescaling it. As a matter of notation, then, write the shifted, rescaled version of $$F$$ as

$$F(\mathcal A;\mu,\sigma) = \Pr\left(\sigma X + \mu\in\mathcal A\right)$$

for $$\mu\in\mathbb{R}^2$$ and $$\sigma\gt 0.$$

The reason for the continuity assumption is that for any $$\mathcal A,$$ it implies the function

$$(\mu,\sigma)\to F(\mathcal{A};\mu,\sigma)$$

is continuous. (Proof: a small change in $$\mu$$ or $$\sigma$$ amounts to applying $$F$$ to a slightly modified version of $$\mathcal A.$$ When $$\mu$$ and $$\sigma$$ change by sufficiently small amounts, the change in the region is so small that--since $$F$$ is continuous--the change in its probability is small, too: that means the map is continuous.)

Consequently the map

$$\Phi: \mathbb{R}^2\times\mathbb{R}^{+}\to \mathbb{R}^3$$

given by the three probabilities

$$\Phi(\mu,\sigma) = (F(\mathcal{R}_1;\mu,\sigma),F(\mathcal{R}_1;\mu,\sigma),F(\mathcal{R}_1;\mu,\sigma))$$

is continuous, too. Because the $$\mathcal{R}_i$$ are a partition, the sum of these three probabilities is always $$1,$$ showing the image of $$\Phi$$ is a subset of the simplex

$$S_2 = \{(x,y,z)\in\mathbb{R}^3\mid x+y+z+1;\ x\ge 0, y\ge 0, z\ge 0\}.$$

From these minimal assumptions we may still deduce a great deal about $$\Phi:$$

• By setting $$\mu$$ to one of the basepoints $$x_i$$ and shrinking $$\sigma,$$ we can focus almost all the probability within $$\mathcal{R}_i.$$ That is, $$\lim_{\sigma\to 0^+} \Phi(x_1,\sigma) = (1,0,0)$$ with a similar statement for $$x_2$$ and $$x_3.$$ Thus, the image of $$\Phi$$ is arbitrarily close to the vertices of the simplex. In fact, the image will include the vertices when the support of $$F$$ is bounded (simply choose $$\sigma$$ smaller than the distance from $$x_i$$ to the boundary of $$\mathcal{R}_i$$ divided by the diameter of the support of $$F$$).

• By following a path $$\gamma_{12}$$ from $$\mathcal{R}_1$$ to $$\mathcal{R}_2$$ that avoids $$\mathcal{R}_3$$ and shrinking $$\sigma$$ sufficiently, we obtain a continuous map $$t \to \Phi(\gamma_{12}(t),\sigma)$$ whose endpoints are arbitrarily close to $$(1,0,0)$$ and $$(0,1,0)$$ (by virtue of the preceding observation) and for which the third component of the image is as small as we may like. Thus, the image of this path is arbitrarily close to the edge of $$S_2$$ from vertex $$(1,0,0)$$ to vertex $$(0,1,0).$$ Since the same construction holds for vertices $$2,3$$ and vertices $$3,1,$$ we conclude the image of $$\Phi$$ is arbitrarily close to the edges of the simplex. (Again, when the support of $$F$$ is bounded, the image of $$\Phi$$ includes the edges.)

It immediately follows from topological considerations that the image of $$\Phi$$ is the entire simplex. (A rigorous proof requires homology theory or homotopy theory, but intuitively it amounts to the idea that if the image of $$\Phi$$ omitted any point in the interior of $$S_2,$$ then $$\Phi$$ would have to be discontinuous--it would have to "tear" its domain--in order to create that hole.)

In particular, any triple of prescribed probabilities $$(p_1,p_2,p_3),$$ being a point of $$S_2,$$ must lie in the closure of the image of $$\Phi$$ (and if none of these probabilities is zero, it must actually be in the image of $$\Phi$$).

This has proven that

For any good partition $$(\mathcal{R}_1,\mathcal{R}_2,\mathcal{R}_3)$$ of the plane, any probability triple $$(p_1,p_2,p_3),$$ and any continuous 2D distribution $$F,$$ there exists a location $$\mu\in\mathbb{R}^2$$ and a scale factor $$\sigma\gt 0$$ for which $$F(\mathcal{R}_i;\mu,\sigma) \approx p_i$$ with to desirable degree of accuracy (and perfect accuracy when all the $$p_i$$ are nonzero or $$F$$ has bounded support).

#
# Find the probability of the angular sector at origin with nonzero oriented
# direction vectors x1 and x2 for a Binormal(mu, Sigma) distribution.
#
psector <- function(origin, x1, x2, mu=c(0,0), Sigma=diag(1,2)) {
require(mvtnorm)
# Are x1 and x2 collinear?
z <- zapsmall(c(sum(x1 * (rev(x2)*c(-1,1))), sqrt(sum(x1^2)*sum(x2^2))), digits=8)[1]
if (z == 0 && sum(x1 * x2) > 0) {
#
# Degenerate sector: interpret as the whole thing.
#
q <- 1
} else {
#
# Shift the mean.
#
mu <- mu - origin
#
# Rotate x1 to (1,0).
#
x1 <- x1 / sqrt(sum(x1^2))
O <- matrix(c(x1[1], -x1[2], x1[2], x1[1]), 2)
Sigma <- O %*% Sigma %*% t(O)
mu <- O %*% mu
x2 <- O %*% x2
#
# Transform x2 to (0,+) if possible, using an area-preserving transformation
#
complement <- isTRUE(sign(x2[2]) == -1) # Sector angle exceeds pi
if (zapsmall(x2)[2] == 0) {             # Sector angle equals pi
lower <- c(-Inf, 0)                   # Upper half plane
upper <- c(Inf, Inf)
} else {
A <- matrix(c(1, 0, -x2[1]/x2[2], sign(x2[2])), 2)
Sigma <- A %*% Sigma %*% t(A)         # Transforms x2 to (0,+) and x1 to (1,0)
mu <- A %*% mu
lower <- c(0, 0)                      # First quadrant
upper <- c(Inf, Inf)
}
q <- pmvnorm(lower=lower, upper=upper, mean=c(mu), sigma=Sigma)
if (complement) q <- 1 - q
}
return(q)
}
#
# Given three sectors originating at origin bounded by directions x1, x2,
# and x3 (oriented positively), along with target probabilities (p1, p2, p3)
# (summing to unity), find mu for which a standard Normal distribution
# centered at mu has the given probabilities in the three sectors.
#
f <- function(p, origin, x1, x2, x3, ...) {
p <- p / sum(p)
mu <- function(x) origin + exp(x[1]) * c(cos(x[2]), sin(x[2]))
obj <- function(x) {
m <- mu(x)
(p[1] - psector(origin, x1, x2, m))^2 +
(p[2] - psector(origin, x2, x3, m))^2 +
(p[3] - psector(origin, x3, x1, m))^2
}
x.hat <- nlm(obj, c(0,0), fscale=1e-14, ...)$estimate mu(x.hat) } # # Example # x1 <- c(1,1) x2 <- c(-1,0) x3 <- c(0,-1) p <- c(0.2, 0.1, 0.7) origin <- c(0,0) mu <- f(p, origin, x1, x2, x3)  • Thanks. May I ask you the following: suppose I set$F$to be the bi-variate standard normal distribution. Hence,$F(\cdot: \mu, \sigma )$is the bi-variate normal distribution with mean$\mu$and variance-covariance matrix$\Sigma\equiv \begin{pmatrix} \sigma^2 & 0 \\ 0 & \sigma^2 \end{pmatrix}$. The off-diagonal elements here can be anything, right? Hence, I can set them to$0$, for simplicity, correct? – TEX Oct 6 '20 at 18:14 • That's basically right, understanding that "anything" really means any value between--but not including--$-\sigma^2$and$\sigma^2.$– whuber Oct 6 '20 at 18:51 • Thanks. May I ask you what is the function$t\in \mathbb{R}\mapsto \gamma_{12}(t)\in \mathbb{R}^2$? This seems to associate to each point in$\mathbb{R}$, a coordinate in$\mathbb{R}^2$that is part of the line$\gamma_{12}$? Sorry, I don't know how to formalise it. – TEX Oct 8 '20 at 10:51 •$\gamma_{12}$is a curve: that is, it's the topological image of an interval$[a,b].$Its endpoints are the image of the endpoints of the interval: namely, the set$\{\gamma_{12}(a),\gamma_{12}(b)\}.$Any such curve that begins and ends deep in the interior of the simplex but wanders close to its boundary along the way, while keeping away from the vertices, will be close to the edge but not the vertices. – whuber Oct 8 '20 at 13:21 • Because the curve is continuous and throughout its range the third component is nearly$0,$in order to connect$(1,0,0)$to$(0,1,0)$it passes through points of the form$(x,y,\epsilon)$where$\epsilon$is tiny: that's an$\epsilon\$ neighborhood of the edge. – whuber Oct 8 '20 at 18:24