$int_{mathbb R^n}e^{{-|x|}^n}dx$












4












$begingroup$


I'm trying to prove that $int_{mathbb R^n}e^{{-|x|}^n}dx = V(B_{n}(0,1))$ using induction and Cavalieri's principle.



Notice that $int_{mathbb R} e^{-|x|}dx = 2 = V(B_1(0, 1))$ so the basis of our induction is true.



Now suppose that for some $k-1$ we have $int_{mathbb R^{k-1}}e^{{-|x|}^{k-1}}dx = V(B_{k-1}(0,1))$



We want to calculate $int_{mathbb R^{k}}e^{-{|x|}^{k}}dx$, we can choose to extract it by spheres since this is a positive function, so this will be equal to $$lim_{r to infty}int_{x_1^2+dots x_k^2 leq r^2}e^{-|x|^{k}}dx.$$



Consider the intersection of this sphere with the plane $x_k = t$. From Cavalieri's principle:



$$
int_{x_1^2+dots x_k^2 leq r^2} e^{-|x|^{k}}dx
= int_{-r}^r
left( int_{x_1^2+dots+x_{k-1}^2 leq r^2-t^2}
e^{-(x_1^2+dots+x_{k-1}^2+t^2)^{k/2}}dx_1dx_2dots dx_{k-1}
right) dt$$



Not sure where to go from here....










share|cite|improve this question











$endgroup$












  • $begingroup$
    Make rather a polar coordinate change?
    $endgroup$
    – Mindlack
    Jan 16 at 16:08










  • $begingroup$
    I don't think that's the intended way. I might be wrong.
    $endgroup$
    – Oria Gruber
    Jan 16 at 16:08
















4












$begingroup$


I'm trying to prove that $int_{mathbb R^n}e^{{-|x|}^n}dx = V(B_{n}(0,1))$ using induction and Cavalieri's principle.



Notice that $int_{mathbb R} e^{-|x|}dx = 2 = V(B_1(0, 1))$ so the basis of our induction is true.



Now suppose that for some $k-1$ we have $int_{mathbb R^{k-1}}e^{{-|x|}^{k-1}}dx = V(B_{k-1}(0,1))$



We want to calculate $int_{mathbb R^{k}}e^{-{|x|}^{k}}dx$, we can choose to extract it by spheres since this is a positive function, so this will be equal to $$lim_{r to infty}int_{x_1^2+dots x_k^2 leq r^2}e^{-|x|^{k}}dx.$$



Consider the intersection of this sphere with the plane $x_k = t$. From Cavalieri's principle:



$$
int_{x_1^2+dots x_k^2 leq r^2} e^{-|x|^{k}}dx
= int_{-r}^r
left( int_{x_1^2+dots+x_{k-1}^2 leq r^2-t^2}
e^{-(x_1^2+dots+x_{k-1}^2+t^2)^{k/2}}dx_1dx_2dots dx_{k-1}
right) dt$$



Not sure where to go from here....










share|cite|improve this question











$endgroup$












  • $begingroup$
    Make rather a polar coordinate change?
    $endgroup$
    – Mindlack
    Jan 16 at 16:08










  • $begingroup$
    I don't think that's the intended way. I might be wrong.
    $endgroup$
    – Oria Gruber
    Jan 16 at 16:08














4












4








4





$begingroup$


I'm trying to prove that $int_{mathbb R^n}e^{{-|x|}^n}dx = V(B_{n}(0,1))$ using induction and Cavalieri's principle.



Notice that $int_{mathbb R} e^{-|x|}dx = 2 = V(B_1(0, 1))$ so the basis of our induction is true.



Now suppose that for some $k-1$ we have $int_{mathbb R^{k-1}}e^{{-|x|}^{k-1}}dx = V(B_{k-1}(0,1))$



We want to calculate $int_{mathbb R^{k}}e^{-{|x|}^{k}}dx$, we can choose to extract it by spheres since this is a positive function, so this will be equal to $$lim_{r to infty}int_{x_1^2+dots x_k^2 leq r^2}e^{-|x|^{k}}dx.$$



Consider the intersection of this sphere with the plane $x_k = t$. From Cavalieri's principle:



$$
int_{x_1^2+dots x_k^2 leq r^2} e^{-|x|^{k}}dx
= int_{-r}^r
left( int_{x_1^2+dots+x_{k-1}^2 leq r^2-t^2}
e^{-(x_1^2+dots+x_{k-1}^2+t^2)^{k/2}}dx_1dx_2dots dx_{k-1}
right) dt$$



Not sure where to go from here....










share|cite|improve this question











$endgroup$




I'm trying to prove that $int_{mathbb R^n}e^{{-|x|}^n}dx = V(B_{n}(0,1))$ using induction and Cavalieri's principle.



Notice that $int_{mathbb R} e^{-|x|}dx = 2 = V(B_1(0, 1))$ so the basis of our induction is true.



Now suppose that for some $k-1$ we have $int_{mathbb R^{k-1}}e^{{-|x|}^{k-1}}dx = V(B_{k-1}(0,1))$



We want to calculate $int_{mathbb R^{k}}e^{-{|x|}^{k}}dx$, we can choose to extract it by spheres since this is a positive function, so this will be equal to $$lim_{r to infty}int_{x_1^2+dots x_k^2 leq r^2}e^{-|x|^{k}}dx.$$



Consider the intersection of this sphere with the plane $x_k = t$. From Cavalieri's principle:



$$
int_{x_1^2+dots x_k^2 leq r^2} e^{-|x|^{k}}dx
= int_{-r}^r
left( int_{x_1^2+dots+x_{k-1}^2 leq r^2-t^2}
e^{-(x_1^2+dots+x_{k-1}^2+t^2)^{k/2}}dx_1dx_2dots dx_{k-1}
right) dt$$



Not sure where to go from here....







calculus integration






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Jan 16 at 16:00









gt6989b

34k22455




34k22455










asked Jan 16 at 15:57









Oria GruberOria Gruber

6,44332460




6,44332460












  • $begingroup$
    Make rather a polar coordinate change?
    $endgroup$
    – Mindlack
    Jan 16 at 16:08










  • $begingroup$
    I don't think that's the intended way. I might be wrong.
    $endgroup$
    – Oria Gruber
    Jan 16 at 16:08


















  • $begingroup$
    Make rather a polar coordinate change?
    $endgroup$
    – Mindlack
    Jan 16 at 16:08










  • $begingroup$
    I don't think that's the intended way. I might be wrong.
    $endgroup$
    – Oria Gruber
    Jan 16 at 16:08
















$begingroup$
Make rather a polar coordinate change?
$endgroup$
– Mindlack
Jan 16 at 16:08




$begingroup$
Make rather a polar coordinate change?
$endgroup$
– Mindlack
Jan 16 at 16:08












$begingroup$
I don't think that's the intended way. I might be wrong.
$endgroup$
– Oria Gruber
Jan 16 at 16:08




$begingroup$
I don't think that's the intended way. I might be wrong.
$endgroup$
– Oria Gruber
Jan 16 at 16:08










1 Answer
1






active

oldest

votes


















3












$begingroup$

I think Cavalieri's principle means this way:
$$
int_{Bbb R^n}e^{-|x|^n}mathrm dx =int_0^1 text{vol}{x:e^{-|x|^n}ge t}mathrm dt.
$$
(Explanation: imagine we are given $z=e^{-x^2-y^2}$. We can calculate the volume under the graph by cutting it along $z=tin (0,1)$, calculating the area of the cross-section and re-gathering it.) This gives
$$begin{eqnarray}
int_0^1 text{vol}{x:e^{-|x|^n}ge t}mathrm dt&=& int_0^1 text{vol}left[(-ln t)^{frac{1}{n}}cdot B_nright]mathrm dt\&=&text{vol}(B_n) int_0^1 (-ln t)mathrm dt\&=&text{vol}(B_n),
end{eqnarray}$$
where $B_n$ is a $n$-dimensional unit ball and $ccdot B_n = {cx;|;xin B_n}=B_n(0,c)$.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Could you elaborate please? Aren't we supposed to look at cross sections? The valid "height" or $x_k$ coordinate for these cross sections is between $(-r,r)$ as $r$ tends to infinity? why $(0,1)$?
    $endgroup$
    – Oria Gruber
    Jan 16 at 16:13










  • $begingroup$
    @OriaGruber Yes, we do the cross section horizontally. Since $0< e^{-|x|^n}le 1$, $0<tle 1$ is sufficient. For example, imagine $z=e^{-x^2-y^2}$. We can calculate the volume under the graph of it by cutting it along $z=c in (0,1)$ and gathering it. I hope this makes it clear.
    $endgroup$
    – Song
    Jan 16 at 16:14












  • $begingroup$
    @OriaGruber You can prove $int_X fdmu = int_0^infty mu({|f| > lambda}) dlambda$ for any measure space $(X,mu)$ and any $f in L^1$, by first proving it for simple functions and then doing linear combinations and taking limits.
    $endgroup$
    – mathworker21
    Jan 16 at 16:19










  • $begingroup$
    @mathworker21 Yes! That is what exactly I did. Thanks for clarification.
    $endgroup$
    – Song
    Jan 16 at 16:20










  • $begingroup$
    May I ask one more question @Song - Why $geq t$? Shouldn't it be equal $t$ since we are looking at a cross section with height $t$?
    $endgroup$
    – Oria Gruber
    Jan 16 at 17:04











Your Answer





StackExchange.ifUsing("editor", function () {
return StackExchange.using("mathjaxEditing", function () {
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
});
});
}, "mathjax-editing");

StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "69"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);

StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});

function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});


}
});














draft saved

draft discarded


















StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3075902%2fint-mathbb-rne-xndx%23new-answer', 'question_page');
}
);

Post as a guest















Required, but never shown

























1 Answer
1






active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









3












$begingroup$

I think Cavalieri's principle means this way:
$$
int_{Bbb R^n}e^{-|x|^n}mathrm dx =int_0^1 text{vol}{x:e^{-|x|^n}ge t}mathrm dt.
$$
(Explanation: imagine we are given $z=e^{-x^2-y^2}$. We can calculate the volume under the graph by cutting it along $z=tin (0,1)$, calculating the area of the cross-section and re-gathering it.) This gives
$$begin{eqnarray}
int_0^1 text{vol}{x:e^{-|x|^n}ge t}mathrm dt&=& int_0^1 text{vol}left[(-ln t)^{frac{1}{n}}cdot B_nright]mathrm dt\&=&text{vol}(B_n) int_0^1 (-ln t)mathrm dt\&=&text{vol}(B_n),
end{eqnarray}$$
where $B_n$ is a $n$-dimensional unit ball and $ccdot B_n = {cx;|;xin B_n}=B_n(0,c)$.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Could you elaborate please? Aren't we supposed to look at cross sections? The valid "height" or $x_k$ coordinate for these cross sections is between $(-r,r)$ as $r$ tends to infinity? why $(0,1)$?
    $endgroup$
    – Oria Gruber
    Jan 16 at 16:13










  • $begingroup$
    @OriaGruber Yes, we do the cross section horizontally. Since $0< e^{-|x|^n}le 1$, $0<tle 1$ is sufficient. For example, imagine $z=e^{-x^2-y^2}$. We can calculate the volume under the graph of it by cutting it along $z=c in (0,1)$ and gathering it. I hope this makes it clear.
    $endgroup$
    – Song
    Jan 16 at 16:14












  • $begingroup$
    @OriaGruber You can prove $int_X fdmu = int_0^infty mu({|f| > lambda}) dlambda$ for any measure space $(X,mu)$ and any $f in L^1$, by first proving it for simple functions and then doing linear combinations and taking limits.
    $endgroup$
    – mathworker21
    Jan 16 at 16:19










  • $begingroup$
    @mathworker21 Yes! That is what exactly I did. Thanks for clarification.
    $endgroup$
    – Song
    Jan 16 at 16:20










  • $begingroup$
    May I ask one more question @Song - Why $geq t$? Shouldn't it be equal $t$ since we are looking at a cross section with height $t$?
    $endgroup$
    – Oria Gruber
    Jan 16 at 17:04
















3












$begingroup$

I think Cavalieri's principle means this way:
$$
int_{Bbb R^n}e^{-|x|^n}mathrm dx =int_0^1 text{vol}{x:e^{-|x|^n}ge t}mathrm dt.
$$
(Explanation: imagine we are given $z=e^{-x^2-y^2}$. We can calculate the volume under the graph by cutting it along $z=tin (0,1)$, calculating the area of the cross-section and re-gathering it.) This gives
$$begin{eqnarray}
int_0^1 text{vol}{x:e^{-|x|^n}ge t}mathrm dt&=& int_0^1 text{vol}left[(-ln t)^{frac{1}{n}}cdot B_nright]mathrm dt\&=&text{vol}(B_n) int_0^1 (-ln t)mathrm dt\&=&text{vol}(B_n),
end{eqnarray}$$
where $B_n$ is a $n$-dimensional unit ball and $ccdot B_n = {cx;|;xin B_n}=B_n(0,c)$.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Could you elaborate please? Aren't we supposed to look at cross sections? The valid "height" or $x_k$ coordinate for these cross sections is between $(-r,r)$ as $r$ tends to infinity? why $(0,1)$?
    $endgroup$
    – Oria Gruber
    Jan 16 at 16:13










  • $begingroup$
    @OriaGruber Yes, we do the cross section horizontally. Since $0< e^{-|x|^n}le 1$, $0<tle 1$ is sufficient. For example, imagine $z=e^{-x^2-y^2}$. We can calculate the volume under the graph of it by cutting it along $z=c in (0,1)$ and gathering it. I hope this makes it clear.
    $endgroup$
    – Song
    Jan 16 at 16:14












  • $begingroup$
    @OriaGruber You can prove $int_X fdmu = int_0^infty mu({|f| > lambda}) dlambda$ for any measure space $(X,mu)$ and any $f in L^1$, by first proving it for simple functions and then doing linear combinations and taking limits.
    $endgroup$
    – mathworker21
    Jan 16 at 16:19










  • $begingroup$
    @mathworker21 Yes! That is what exactly I did. Thanks for clarification.
    $endgroup$
    – Song
    Jan 16 at 16:20










  • $begingroup$
    May I ask one more question @Song - Why $geq t$? Shouldn't it be equal $t$ since we are looking at a cross section with height $t$?
    $endgroup$
    – Oria Gruber
    Jan 16 at 17:04














3












3








3





$begingroup$

I think Cavalieri's principle means this way:
$$
int_{Bbb R^n}e^{-|x|^n}mathrm dx =int_0^1 text{vol}{x:e^{-|x|^n}ge t}mathrm dt.
$$
(Explanation: imagine we are given $z=e^{-x^2-y^2}$. We can calculate the volume under the graph by cutting it along $z=tin (0,1)$, calculating the area of the cross-section and re-gathering it.) This gives
$$begin{eqnarray}
int_0^1 text{vol}{x:e^{-|x|^n}ge t}mathrm dt&=& int_0^1 text{vol}left[(-ln t)^{frac{1}{n}}cdot B_nright]mathrm dt\&=&text{vol}(B_n) int_0^1 (-ln t)mathrm dt\&=&text{vol}(B_n),
end{eqnarray}$$
where $B_n$ is a $n$-dimensional unit ball and $ccdot B_n = {cx;|;xin B_n}=B_n(0,c)$.






share|cite|improve this answer











$endgroup$



I think Cavalieri's principle means this way:
$$
int_{Bbb R^n}e^{-|x|^n}mathrm dx =int_0^1 text{vol}{x:e^{-|x|^n}ge t}mathrm dt.
$$
(Explanation: imagine we are given $z=e^{-x^2-y^2}$. We can calculate the volume under the graph by cutting it along $z=tin (0,1)$, calculating the area of the cross-section and re-gathering it.) This gives
$$begin{eqnarray}
int_0^1 text{vol}{x:e^{-|x|^n}ge t}mathrm dt&=& int_0^1 text{vol}left[(-ln t)^{frac{1}{n}}cdot B_nright]mathrm dt\&=&text{vol}(B_n) int_0^1 (-ln t)mathrm dt\&=&text{vol}(B_n),
end{eqnarray}$$
where $B_n$ is a $n$-dimensional unit ball and $ccdot B_n = {cx;|;xin B_n}=B_n(0,c)$.







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Jan 16 at 16:21

























answered Jan 16 at 16:09









SongSong

12.2k630




12.2k630












  • $begingroup$
    Could you elaborate please? Aren't we supposed to look at cross sections? The valid "height" or $x_k$ coordinate for these cross sections is between $(-r,r)$ as $r$ tends to infinity? why $(0,1)$?
    $endgroup$
    – Oria Gruber
    Jan 16 at 16:13










  • $begingroup$
    @OriaGruber Yes, we do the cross section horizontally. Since $0< e^{-|x|^n}le 1$, $0<tle 1$ is sufficient. For example, imagine $z=e^{-x^2-y^2}$. We can calculate the volume under the graph of it by cutting it along $z=c in (0,1)$ and gathering it. I hope this makes it clear.
    $endgroup$
    – Song
    Jan 16 at 16:14












  • $begingroup$
    @OriaGruber You can prove $int_X fdmu = int_0^infty mu({|f| > lambda}) dlambda$ for any measure space $(X,mu)$ and any $f in L^1$, by first proving it for simple functions and then doing linear combinations and taking limits.
    $endgroup$
    – mathworker21
    Jan 16 at 16:19










  • $begingroup$
    @mathworker21 Yes! That is what exactly I did. Thanks for clarification.
    $endgroup$
    – Song
    Jan 16 at 16:20










  • $begingroup$
    May I ask one more question @Song - Why $geq t$? Shouldn't it be equal $t$ since we are looking at a cross section with height $t$?
    $endgroup$
    – Oria Gruber
    Jan 16 at 17:04


















  • $begingroup$
    Could you elaborate please? Aren't we supposed to look at cross sections? The valid "height" or $x_k$ coordinate for these cross sections is between $(-r,r)$ as $r$ tends to infinity? why $(0,1)$?
    $endgroup$
    – Oria Gruber
    Jan 16 at 16:13










  • $begingroup$
    @OriaGruber Yes, we do the cross section horizontally. Since $0< e^{-|x|^n}le 1$, $0<tle 1$ is sufficient. For example, imagine $z=e^{-x^2-y^2}$. We can calculate the volume under the graph of it by cutting it along $z=c in (0,1)$ and gathering it. I hope this makes it clear.
    $endgroup$
    – Song
    Jan 16 at 16:14












  • $begingroup$
    @OriaGruber You can prove $int_X fdmu = int_0^infty mu({|f| > lambda}) dlambda$ for any measure space $(X,mu)$ and any $f in L^1$, by first proving it for simple functions and then doing linear combinations and taking limits.
    $endgroup$
    – mathworker21
    Jan 16 at 16:19










  • $begingroup$
    @mathworker21 Yes! That is what exactly I did. Thanks for clarification.
    $endgroup$
    – Song
    Jan 16 at 16:20










  • $begingroup$
    May I ask one more question @Song - Why $geq t$? Shouldn't it be equal $t$ since we are looking at a cross section with height $t$?
    $endgroup$
    – Oria Gruber
    Jan 16 at 17:04
















$begingroup$
Could you elaborate please? Aren't we supposed to look at cross sections? The valid "height" or $x_k$ coordinate for these cross sections is between $(-r,r)$ as $r$ tends to infinity? why $(0,1)$?
$endgroup$
– Oria Gruber
Jan 16 at 16:13




$begingroup$
Could you elaborate please? Aren't we supposed to look at cross sections? The valid "height" or $x_k$ coordinate for these cross sections is between $(-r,r)$ as $r$ tends to infinity? why $(0,1)$?
$endgroup$
– Oria Gruber
Jan 16 at 16:13












$begingroup$
@OriaGruber Yes, we do the cross section horizontally. Since $0< e^{-|x|^n}le 1$, $0<tle 1$ is sufficient. For example, imagine $z=e^{-x^2-y^2}$. We can calculate the volume under the graph of it by cutting it along $z=c in (0,1)$ and gathering it. I hope this makes it clear.
$endgroup$
– Song
Jan 16 at 16:14






$begingroup$
@OriaGruber Yes, we do the cross section horizontally. Since $0< e^{-|x|^n}le 1$, $0<tle 1$ is sufficient. For example, imagine $z=e^{-x^2-y^2}$. We can calculate the volume under the graph of it by cutting it along $z=c in (0,1)$ and gathering it. I hope this makes it clear.
$endgroup$
– Song
Jan 16 at 16:14














$begingroup$
@OriaGruber You can prove $int_X fdmu = int_0^infty mu({|f| > lambda}) dlambda$ for any measure space $(X,mu)$ and any $f in L^1$, by first proving it for simple functions and then doing linear combinations and taking limits.
$endgroup$
– mathworker21
Jan 16 at 16:19




$begingroup$
@OriaGruber You can prove $int_X fdmu = int_0^infty mu({|f| > lambda}) dlambda$ for any measure space $(X,mu)$ and any $f in L^1$, by first proving it for simple functions and then doing linear combinations and taking limits.
$endgroup$
– mathworker21
Jan 16 at 16:19












$begingroup$
@mathworker21 Yes! That is what exactly I did. Thanks for clarification.
$endgroup$
– Song
Jan 16 at 16:20




$begingroup$
@mathworker21 Yes! That is what exactly I did. Thanks for clarification.
$endgroup$
– Song
Jan 16 at 16:20












$begingroup$
May I ask one more question @Song - Why $geq t$? Shouldn't it be equal $t$ since we are looking at a cross section with height $t$?
$endgroup$
– Oria Gruber
Jan 16 at 17:04




$begingroup$
May I ask one more question @Song - Why $geq t$? Shouldn't it be equal $t$ since we are looking at a cross section with height $t$?
$endgroup$
– Oria Gruber
Jan 16 at 17:04


















draft saved

draft discarded




















































Thanks for contributing an answer to Mathematics Stack Exchange!


  • Please be sure to answer the question. Provide details and share your research!

But avoid



  • Asking for help, clarification, or responding to other answers.

  • Making statements based on opinion; back them up with references or personal experience.


Use MathJax to format equations. MathJax reference.


To learn more, see our tips on writing great answers.




draft saved


draft discarded














StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3075902%2fint-mathbb-rne-xndx%23new-answer', 'question_page');
}
);

Post as a guest















Required, but never shown





















































Required, but never shown














Required, but never shown












Required, but never shown







Required, but never shown

































Required, but never shown














Required, but never shown












Required, but never shown







Required, but never shown







Popular posts from this blog

Mario Kart Wii

The Binding of Isaac: Rebirth/Afterbirth

What does “Dominus providebit” mean?