The $ell_2$ norm of two stochastic vectors
$begingroup$
I would like to know why this is true:
Given an $n$-dimensional stochastic vector $v$ and another $n$-dimensional stochastic vector $u$ which distributes uniformly over ${1,dots,n}$ (meaning if the length of the vector $u$ is $n=3$ then $u = (1/3,1/3,1/3)$) then $lVert u-vrVert_2 leq 1$.
For example, let $v = (0.8,0.2)$ and $v = (0.5,0.5)$: $lVert u-vrVert_2 = lVert(0.3,-0.3)rVert_2=sqrt{2cdot0.3^2}$
which is less than 1.
I tried many different inputs and it works for all of them but I can’t figure out why?
inequality probability-distributions vectors stochastic-calculus
$endgroup$
|
show 1 more comment
$begingroup$
I would like to know why this is true:
Given an $n$-dimensional stochastic vector $v$ and another $n$-dimensional stochastic vector $u$ which distributes uniformly over ${1,dots,n}$ (meaning if the length of the vector $u$ is $n=3$ then $u = (1/3,1/3,1/3)$) then $lVert u-vrVert_2 leq 1$.
For example, let $v = (0.8,0.2)$ and $v = (0.5,0.5)$: $lVert u-vrVert_2 = lVert(0.3,-0.3)rVert_2=sqrt{2cdot0.3^2}$
which is less than 1.
I tried many different inputs and it works for all of them but I can’t figure out why?
inequality probability-distributions vectors stochastic-calculus
$endgroup$
$begingroup$
Your statement is too confusing. Are you asking about 2-d or 3-d vectors?
$endgroup$
– herb steinberg
Dec 16 '18 at 18:07
$begingroup$
@herbsteinberg I am asking for n-d vectors. the 2-d and 3-d vectors where just an example in order to simplify the problem.
$endgroup$
– נירייב שמואל
Dec 16 '18 at 18:09
1
$begingroup$
There is other confusing stuff. For example U length 3 and U=(1/3,1/3,1/3) is wrong. In general $|v-u|le 1$ is not true without more descriptions for $u$ and $v$.
$endgroup$
– herb steinberg
Dec 16 '18 at 18:17
$begingroup$
@herbsteinberg that's why I indicated that U distribute uniformly between 1 untill n.
$endgroup$
– נירייב שמואל
Dec 16 '18 at 18:20
$begingroup$
If I may, why are there votes to close with no comment on it? Sure, the OP hasnt included their attempts (if any), but then the minimum here (for a new contributor) would be to point it out instead of just voting for closure... "נירייב שמואל is a new contributor to this site. Take care in asking for clarification, commenting, and answering."
$endgroup$
– Clement C.
Dec 17 '18 at 21:32
|
show 1 more comment
$begingroup$
I would like to know why this is true:
Given an $n$-dimensional stochastic vector $v$ and another $n$-dimensional stochastic vector $u$ which distributes uniformly over ${1,dots,n}$ (meaning if the length of the vector $u$ is $n=3$ then $u = (1/3,1/3,1/3)$) then $lVert u-vrVert_2 leq 1$.
For example, let $v = (0.8,0.2)$ and $v = (0.5,0.5)$: $lVert u-vrVert_2 = lVert(0.3,-0.3)rVert_2=sqrt{2cdot0.3^2}$
which is less than 1.
I tried many different inputs and it works for all of them but I can’t figure out why?
inequality probability-distributions vectors stochastic-calculus
$endgroup$
I would like to know why this is true:
Given an $n$-dimensional stochastic vector $v$ and another $n$-dimensional stochastic vector $u$ which distributes uniformly over ${1,dots,n}$ (meaning if the length of the vector $u$ is $n=3$ then $u = (1/3,1/3,1/3)$) then $lVert u-vrVert_2 leq 1$.
For example, let $v = (0.8,0.2)$ and $v = (0.5,0.5)$: $lVert u-vrVert_2 = lVert(0.3,-0.3)rVert_2=sqrt{2cdot0.3^2}$
which is less than 1.
I tried many different inputs and it works for all of them but I can’t figure out why?
inequality probability-distributions vectors stochastic-calculus
inequality probability-distributions vectors stochastic-calculus
edited Dec 16 '18 at 18:52
Clement C.
50.9k33992
50.9k33992
asked Dec 16 '18 at 17:49
נירייב שמואלנירייב שמואל
195
195
$begingroup$
Your statement is too confusing. Are you asking about 2-d or 3-d vectors?
$endgroup$
– herb steinberg
Dec 16 '18 at 18:07
$begingroup$
@herbsteinberg I am asking for n-d vectors. the 2-d and 3-d vectors where just an example in order to simplify the problem.
$endgroup$
– נירייב שמואל
Dec 16 '18 at 18:09
1
$begingroup$
There is other confusing stuff. For example U length 3 and U=(1/3,1/3,1/3) is wrong. In general $|v-u|le 1$ is not true without more descriptions for $u$ and $v$.
$endgroup$
– herb steinberg
Dec 16 '18 at 18:17
$begingroup$
@herbsteinberg that's why I indicated that U distribute uniformly between 1 untill n.
$endgroup$
– נירייב שמואל
Dec 16 '18 at 18:20
$begingroup$
If I may, why are there votes to close with no comment on it? Sure, the OP hasnt included their attempts (if any), but then the minimum here (for a new contributor) would be to point it out instead of just voting for closure... "נירייב שמואל is a new contributor to this site. Take care in asking for clarification, commenting, and answering."
$endgroup$
– Clement C.
Dec 17 '18 at 21:32
|
show 1 more comment
$begingroup$
Your statement is too confusing. Are you asking about 2-d or 3-d vectors?
$endgroup$
– herb steinberg
Dec 16 '18 at 18:07
$begingroup$
@herbsteinberg I am asking for n-d vectors. the 2-d and 3-d vectors where just an example in order to simplify the problem.
$endgroup$
– נירייב שמואל
Dec 16 '18 at 18:09
1
$begingroup$
There is other confusing stuff. For example U length 3 and U=(1/3,1/3,1/3) is wrong. In general $|v-u|le 1$ is not true without more descriptions for $u$ and $v$.
$endgroup$
– herb steinberg
Dec 16 '18 at 18:17
$begingroup$
@herbsteinberg that's why I indicated that U distribute uniformly between 1 untill n.
$endgroup$
– נירייב שמואל
Dec 16 '18 at 18:20
$begingroup$
If I may, why are there votes to close with no comment on it? Sure, the OP hasnt included their attempts (if any), but then the minimum here (for a new contributor) would be to point it out instead of just voting for closure... "נירייב שמואל is a new contributor to this site. Take care in asking for clarification, commenting, and answering."
$endgroup$
– Clement C.
Dec 17 '18 at 21:32
$begingroup$
Your statement is too confusing. Are you asking about 2-d or 3-d vectors?
$endgroup$
– herb steinberg
Dec 16 '18 at 18:07
$begingroup$
Your statement is too confusing. Are you asking about 2-d or 3-d vectors?
$endgroup$
– herb steinberg
Dec 16 '18 at 18:07
$begingroup$
@herbsteinberg I am asking for n-d vectors. the 2-d and 3-d vectors where just an example in order to simplify the problem.
$endgroup$
– נירייב שמואל
Dec 16 '18 at 18:09
$begingroup$
@herbsteinberg I am asking for n-d vectors. the 2-d and 3-d vectors where just an example in order to simplify the problem.
$endgroup$
– נירייב שמואל
Dec 16 '18 at 18:09
1
1
$begingroup$
There is other confusing stuff. For example U length 3 and U=(1/3,1/3,1/3) is wrong. In general $|v-u|le 1$ is not true without more descriptions for $u$ and $v$.
$endgroup$
– herb steinberg
Dec 16 '18 at 18:17
$begingroup$
There is other confusing stuff. For example U length 3 and U=(1/3,1/3,1/3) is wrong. In general $|v-u|le 1$ is not true without more descriptions for $u$ and $v$.
$endgroup$
– herb steinberg
Dec 16 '18 at 18:17
$begingroup$
@herbsteinberg that's why I indicated that U distribute uniformly between 1 untill n.
$endgroup$
– נירייב שמואל
Dec 16 '18 at 18:20
$begingroup$
@herbsteinberg that's why I indicated that U distribute uniformly between 1 untill n.
$endgroup$
– נירייב שמואל
Dec 16 '18 at 18:20
$begingroup$
If I may, why are there votes to close with no comment on it? Sure, the OP hasnt included their attempts (if any), but then the minimum here (for a new contributor) would be to point it out instead of just voting for closure... "נירייב שמואל is a new contributor to this site. Take care in asking for clarification, commenting, and answering."
$endgroup$
– Clement C.
Dec 17 '18 at 21:32
$begingroup$
If I may, why are there votes to close with no comment on it? Sure, the OP hasnt included their attempts (if any), but then the minimum here (for a new contributor) would be to point it out instead of just voting for closure... "נירייב שמואל is a new contributor to this site. Take care in asking for clarification, commenting, and answering."
$endgroup$
– Clement C.
Dec 17 '18 at 21:32
|
show 1 more comment
1 Answer
1
active
oldest
votes
$begingroup$
First, you can show it will be at most 2 by the triangle inequality, and even a bit better: for $u,v$ two $n$-dimensional vectors in the probability simplex with $u$ being uniform,
$$
lVert u-vrVert_2 leq lVert urVert_2+ lVert vrVert_2 = frac{1}{sqrt{n}} + 1
$$
Now, for the stronger claim. Observe that
$$begin{align}
lVert u-vrVert_2^2 &= sum_{k=1}^n (u_k-v_k)^2 =
sum_{k=1}^n u_k^2 + sum_{k=1}^n v_k^2 - 2sum_{k=1}^n u_k v_k
= ncdot frac{1}{n^2} + lVert vrVert_2^2 - frac{2}{n}sum_{k=1}^n v_k\
&= frac{1}{n} + lVert vrVert_2^2 - frac{2}{n}
= lVert vrVert_2^2 - frac{1}{n} \
&leq 1 - frac{1}{n}
end{align}$$
where the last inequality comes from the fact that $$lVert vrVert_2^2 = sum_{k=1}^n v_k^2 leq sum_{k=1}^n v_k = 1$$
(more generally, $lVert vrVert_p leq lVert vrVert_q$ if $pgeq qgeq 1$).
Upshot: the squared $ell_2$ distance of a probability vector to uniformity is equal, up to an additive factor $1/n$, to its squared $ell_2$ norm:
$$
lVert u-vrVert_2^2 = lVert vrVert_2^2 - frac{1}{n}
$$
which can itself be interpreted as the collision probability
$$
lVert vrVert_2^2 = mathbb{P}_{v}{ X=Y}
$$
where $X,Y$ are independent random variables with pmf $v$.
$endgroup$
$begingroup$
Note: the reason for the last comment in the "upshot" is that this view of the $ell_2$ distance to uniform as directly related to the collision probability is the basis for most uniformity testing algorithms (given i.i.d. realizations from a discrete r.v., decide whether its distribution is uniform vs. far from uniform). I figured it was worth mentioning.
$endgroup$
– Clement C.
Dec 16 '18 at 18:49
add a comment |
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1 Answer
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1 Answer
1
active
oldest
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active
oldest
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active
oldest
votes
$begingroup$
First, you can show it will be at most 2 by the triangle inequality, and even a bit better: for $u,v$ two $n$-dimensional vectors in the probability simplex with $u$ being uniform,
$$
lVert u-vrVert_2 leq lVert urVert_2+ lVert vrVert_2 = frac{1}{sqrt{n}} + 1
$$
Now, for the stronger claim. Observe that
$$begin{align}
lVert u-vrVert_2^2 &= sum_{k=1}^n (u_k-v_k)^2 =
sum_{k=1}^n u_k^2 + sum_{k=1}^n v_k^2 - 2sum_{k=1}^n u_k v_k
= ncdot frac{1}{n^2} + lVert vrVert_2^2 - frac{2}{n}sum_{k=1}^n v_k\
&= frac{1}{n} + lVert vrVert_2^2 - frac{2}{n}
= lVert vrVert_2^2 - frac{1}{n} \
&leq 1 - frac{1}{n}
end{align}$$
where the last inequality comes from the fact that $$lVert vrVert_2^2 = sum_{k=1}^n v_k^2 leq sum_{k=1}^n v_k = 1$$
(more generally, $lVert vrVert_p leq lVert vrVert_q$ if $pgeq qgeq 1$).
Upshot: the squared $ell_2$ distance of a probability vector to uniformity is equal, up to an additive factor $1/n$, to its squared $ell_2$ norm:
$$
lVert u-vrVert_2^2 = lVert vrVert_2^2 - frac{1}{n}
$$
which can itself be interpreted as the collision probability
$$
lVert vrVert_2^2 = mathbb{P}_{v}{ X=Y}
$$
where $X,Y$ are independent random variables with pmf $v$.
$endgroup$
$begingroup$
Note: the reason for the last comment in the "upshot" is that this view of the $ell_2$ distance to uniform as directly related to the collision probability is the basis for most uniformity testing algorithms (given i.i.d. realizations from a discrete r.v., decide whether its distribution is uniform vs. far from uniform). I figured it was worth mentioning.
$endgroup$
– Clement C.
Dec 16 '18 at 18:49
add a comment |
$begingroup$
First, you can show it will be at most 2 by the triangle inequality, and even a bit better: for $u,v$ two $n$-dimensional vectors in the probability simplex with $u$ being uniform,
$$
lVert u-vrVert_2 leq lVert urVert_2+ lVert vrVert_2 = frac{1}{sqrt{n}} + 1
$$
Now, for the stronger claim. Observe that
$$begin{align}
lVert u-vrVert_2^2 &= sum_{k=1}^n (u_k-v_k)^2 =
sum_{k=1}^n u_k^2 + sum_{k=1}^n v_k^2 - 2sum_{k=1}^n u_k v_k
= ncdot frac{1}{n^2} + lVert vrVert_2^2 - frac{2}{n}sum_{k=1}^n v_k\
&= frac{1}{n} + lVert vrVert_2^2 - frac{2}{n}
= lVert vrVert_2^2 - frac{1}{n} \
&leq 1 - frac{1}{n}
end{align}$$
where the last inequality comes from the fact that $$lVert vrVert_2^2 = sum_{k=1}^n v_k^2 leq sum_{k=1}^n v_k = 1$$
(more generally, $lVert vrVert_p leq lVert vrVert_q$ if $pgeq qgeq 1$).
Upshot: the squared $ell_2$ distance of a probability vector to uniformity is equal, up to an additive factor $1/n$, to its squared $ell_2$ norm:
$$
lVert u-vrVert_2^2 = lVert vrVert_2^2 - frac{1}{n}
$$
which can itself be interpreted as the collision probability
$$
lVert vrVert_2^2 = mathbb{P}_{v}{ X=Y}
$$
where $X,Y$ are independent random variables with pmf $v$.
$endgroup$
$begingroup$
Note: the reason for the last comment in the "upshot" is that this view of the $ell_2$ distance to uniform as directly related to the collision probability is the basis for most uniformity testing algorithms (given i.i.d. realizations from a discrete r.v., decide whether its distribution is uniform vs. far from uniform). I figured it was worth mentioning.
$endgroup$
– Clement C.
Dec 16 '18 at 18:49
add a comment |
$begingroup$
First, you can show it will be at most 2 by the triangle inequality, and even a bit better: for $u,v$ two $n$-dimensional vectors in the probability simplex with $u$ being uniform,
$$
lVert u-vrVert_2 leq lVert urVert_2+ lVert vrVert_2 = frac{1}{sqrt{n}} + 1
$$
Now, for the stronger claim. Observe that
$$begin{align}
lVert u-vrVert_2^2 &= sum_{k=1}^n (u_k-v_k)^2 =
sum_{k=1}^n u_k^2 + sum_{k=1}^n v_k^2 - 2sum_{k=1}^n u_k v_k
= ncdot frac{1}{n^2} + lVert vrVert_2^2 - frac{2}{n}sum_{k=1}^n v_k\
&= frac{1}{n} + lVert vrVert_2^2 - frac{2}{n}
= lVert vrVert_2^2 - frac{1}{n} \
&leq 1 - frac{1}{n}
end{align}$$
where the last inequality comes from the fact that $$lVert vrVert_2^2 = sum_{k=1}^n v_k^2 leq sum_{k=1}^n v_k = 1$$
(more generally, $lVert vrVert_p leq lVert vrVert_q$ if $pgeq qgeq 1$).
Upshot: the squared $ell_2$ distance of a probability vector to uniformity is equal, up to an additive factor $1/n$, to its squared $ell_2$ norm:
$$
lVert u-vrVert_2^2 = lVert vrVert_2^2 - frac{1}{n}
$$
which can itself be interpreted as the collision probability
$$
lVert vrVert_2^2 = mathbb{P}_{v}{ X=Y}
$$
where $X,Y$ are independent random variables with pmf $v$.
$endgroup$
First, you can show it will be at most 2 by the triangle inequality, and even a bit better: for $u,v$ two $n$-dimensional vectors in the probability simplex with $u$ being uniform,
$$
lVert u-vrVert_2 leq lVert urVert_2+ lVert vrVert_2 = frac{1}{sqrt{n}} + 1
$$
Now, for the stronger claim. Observe that
$$begin{align}
lVert u-vrVert_2^2 &= sum_{k=1}^n (u_k-v_k)^2 =
sum_{k=1}^n u_k^2 + sum_{k=1}^n v_k^2 - 2sum_{k=1}^n u_k v_k
= ncdot frac{1}{n^2} + lVert vrVert_2^2 - frac{2}{n}sum_{k=1}^n v_k\
&= frac{1}{n} + lVert vrVert_2^2 - frac{2}{n}
= lVert vrVert_2^2 - frac{1}{n} \
&leq 1 - frac{1}{n}
end{align}$$
where the last inequality comes from the fact that $$lVert vrVert_2^2 = sum_{k=1}^n v_k^2 leq sum_{k=1}^n v_k = 1$$
(more generally, $lVert vrVert_p leq lVert vrVert_q$ if $pgeq qgeq 1$).
Upshot: the squared $ell_2$ distance of a probability vector to uniformity is equal, up to an additive factor $1/n$, to its squared $ell_2$ norm:
$$
lVert u-vrVert_2^2 = lVert vrVert_2^2 - frac{1}{n}
$$
which can itself be interpreted as the collision probability
$$
lVert vrVert_2^2 = mathbb{P}_{v}{ X=Y}
$$
where $X,Y$ are independent random variables with pmf $v$.
edited Dec 17 '18 at 0:07
answered Dec 16 '18 at 18:34
Clement C.Clement C.
50.9k33992
50.9k33992
$begingroup$
Note: the reason for the last comment in the "upshot" is that this view of the $ell_2$ distance to uniform as directly related to the collision probability is the basis for most uniformity testing algorithms (given i.i.d. realizations from a discrete r.v., decide whether its distribution is uniform vs. far from uniform). I figured it was worth mentioning.
$endgroup$
– Clement C.
Dec 16 '18 at 18:49
add a comment |
$begingroup$
Note: the reason for the last comment in the "upshot" is that this view of the $ell_2$ distance to uniform as directly related to the collision probability is the basis for most uniformity testing algorithms (given i.i.d. realizations from a discrete r.v., decide whether its distribution is uniform vs. far from uniform). I figured it was worth mentioning.
$endgroup$
– Clement C.
Dec 16 '18 at 18:49
$begingroup$
Note: the reason for the last comment in the "upshot" is that this view of the $ell_2$ distance to uniform as directly related to the collision probability is the basis for most uniformity testing algorithms (given i.i.d. realizations from a discrete r.v., decide whether its distribution is uniform vs. far from uniform). I figured it was worth mentioning.
$endgroup$
– Clement C.
Dec 16 '18 at 18:49
$begingroup$
Note: the reason for the last comment in the "upshot" is that this view of the $ell_2$ distance to uniform as directly related to the collision probability is the basis for most uniformity testing algorithms (given i.i.d. realizations from a discrete r.v., decide whether its distribution is uniform vs. far from uniform). I figured it was worth mentioning.
$endgroup$
– Clement C.
Dec 16 '18 at 18:49
add a comment |
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$begingroup$
Your statement is too confusing. Are you asking about 2-d or 3-d vectors?
$endgroup$
– herb steinberg
Dec 16 '18 at 18:07
$begingroup$
@herbsteinberg I am asking for n-d vectors. the 2-d and 3-d vectors where just an example in order to simplify the problem.
$endgroup$
– נירייב שמואל
Dec 16 '18 at 18:09
1
$begingroup$
There is other confusing stuff. For example U length 3 and U=(1/3,1/3,1/3) is wrong. In general $|v-u|le 1$ is not true without more descriptions for $u$ and $v$.
$endgroup$
– herb steinberg
Dec 16 '18 at 18:17
$begingroup$
@herbsteinberg that's why I indicated that U distribute uniformly between 1 untill n.
$endgroup$
– נירייב שמואל
Dec 16 '18 at 18:20
$begingroup$
If I may, why are there votes to close with no comment on it? Sure, the OP hasnt included their attempts (if any), but then the minimum here (for a new contributor) would be to point it out instead of just voting for closure... "נירייב שמואל is a new contributor to this site. Take care in asking for clarification, commenting, and answering."
$endgroup$
– Clement C.
Dec 17 '18 at 21:32