$t=exp(x-u) quad u=exp(x-t)$ for some $x =0..1$ Can $u ne t$?
$begingroup$
From deeper discussion of this question I have the pair of equations
$$ t = exp(x - u) \
u = exp(x - t) $$
Rearranging
$$ t/u = exp(t)/exp(u) qquad text{ or }\
u/exp(u) = t/exp(t) $$
I'm looking at $0 le x le 1$ if this is simpler. My hypothese is that
$t=u$ is required.
Trying with arithmetic or geometric mean of $u$ and $t$ didn't lead easily to a solution, also the use of the taylor-series for $exp()$ didn't give me the idea.
I think however it must be somehow easy to be proved, but after a time fiddling with it the penny still didn't drop...
Q: does the hypothese hold that for all $x$ in the interval $t=u$ is required?
algebra-precalculus elementary-number-theory
$endgroup$
add a comment |
$begingroup$
From deeper discussion of this question I have the pair of equations
$$ t = exp(x - u) \
u = exp(x - t) $$
Rearranging
$$ t/u = exp(t)/exp(u) qquad text{ or }\
u/exp(u) = t/exp(t) $$
I'm looking at $0 le x le 1$ if this is simpler. My hypothese is that
$t=u$ is required.
Trying with arithmetic or geometric mean of $u$ and $t$ didn't lead easily to a solution, also the use of the taylor-series for $exp()$ didn't give me the idea.
I think however it must be somehow easy to be proved, but after a time fiddling with it the penny still didn't drop...
Q: does the hypothese hold that for all $x$ in the interval $t=u$ is required?
algebra-precalculus elementary-number-theory
$endgroup$
$begingroup$
@coffeemath, you have the right idea but, unless I misread the question, you should be using the reciprocal of the function you specify.
$endgroup$
– John Omielan
Dec 24 '18 at 0:16
$begingroup$
@JohnOmielan But $f$ is 1-1 iff $1/f$ is.
$endgroup$
– coffeemath
Dec 24 '18 at 0:17
1
$begingroup$
@coffeemath Thanks for explaining. You are right. I did not think this is what you were doing.
$endgroup$
– John Omielan
Dec 24 '18 at 0:19
add a comment |
$begingroup$
From deeper discussion of this question I have the pair of equations
$$ t = exp(x - u) \
u = exp(x - t) $$
Rearranging
$$ t/u = exp(t)/exp(u) qquad text{ or }\
u/exp(u) = t/exp(t) $$
I'm looking at $0 le x le 1$ if this is simpler. My hypothese is that
$t=u$ is required.
Trying with arithmetic or geometric mean of $u$ and $t$ didn't lead easily to a solution, also the use of the taylor-series for $exp()$ didn't give me the idea.
I think however it must be somehow easy to be proved, but after a time fiddling with it the penny still didn't drop...
Q: does the hypothese hold that for all $x$ in the interval $t=u$ is required?
algebra-precalculus elementary-number-theory
$endgroup$
From deeper discussion of this question I have the pair of equations
$$ t = exp(x - u) \
u = exp(x - t) $$
Rearranging
$$ t/u = exp(t)/exp(u) qquad text{ or }\
u/exp(u) = t/exp(t) $$
I'm looking at $0 le x le 1$ if this is simpler. My hypothese is that
$t=u$ is required.
Trying with arithmetic or geometric mean of $u$ and $t$ didn't lead easily to a solution, also the use of the taylor-series for $exp()$ didn't give me the idea.
I think however it must be somehow easy to be proved, but after a time fiddling with it the penny still didn't drop...
Q: does the hypothese hold that for all $x$ in the interval $t=u$ is required?
algebra-precalculus elementary-number-theory
algebra-precalculus elementary-number-theory
asked Dec 24 '18 at 0:03
Gottfried HelmsGottfried Helms
23.4k24598
23.4k24598
$begingroup$
@coffeemath, you have the right idea but, unless I misread the question, you should be using the reciprocal of the function you specify.
$endgroup$
– John Omielan
Dec 24 '18 at 0:16
$begingroup$
@JohnOmielan But $f$ is 1-1 iff $1/f$ is.
$endgroup$
– coffeemath
Dec 24 '18 at 0:17
1
$begingroup$
@coffeemath Thanks for explaining. You are right. I did not think this is what you were doing.
$endgroup$
– John Omielan
Dec 24 '18 at 0:19
add a comment |
$begingroup$
@coffeemath, you have the right idea but, unless I misread the question, you should be using the reciprocal of the function you specify.
$endgroup$
– John Omielan
Dec 24 '18 at 0:16
$begingroup$
@JohnOmielan But $f$ is 1-1 iff $1/f$ is.
$endgroup$
– coffeemath
Dec 24 '18 at 0:17
1
$begingroup$
@coffeemath Thanks for explaining. You are right. I did not think this is what you were doing.
$endgroup$
– John Omielan
Dec 24 '18 at 0:19
$begingroup$
@coffeemath, you have the right idea but, unless I misread the question, you should be using the reciprocal of the function you specify.
$endgroup$
– John Omielan
Dec 24 '18 at 0:16
$begingroup$
@coffeemath, you have the right idea but, unless I misread the question, you should be using the reciprocal of the function you specify.
$endgroup$
– John Omielan
Dec 24 '18 at 0:16
$begingroup$
@JohnOmielan But $f$ is 1-1 iff $1/f$ is.
$endgroup$
– coffeemath
Dec 24 '18 at 0:17
$begingroup$
@JohnOmielan But $f$ is 1-1 iff $1/f$ is.
$endgroup$
– coffeemath
Dec 24 '18 at 0:17
1
1
$begingroup$
@coffeemath Thanks for explaining. You are right. I did not think this is what you were doing.
$endgroup$
– John Omielan
Dec 24 '18 at 0:19
$begingroup$
@coffeemath Thanks for explaining. You are right. I did not think this is what you were doing.
$endgroup$
– John Omielan
Dec 24 '18 at 0:19
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Rearranging, you get:
$$ tag{1} te^u = e^x = ue^t $$
and from there
$$ tag{2} frac{t}{e^t} = frac{u}{e^u} $$
The function $tmapsto t/e^t$ increases from $t=0$ to $t=1$, but after that it starts falling again -- so you can find $tne u$ that satisfies $(2)$ quite fine, and from there use $(1)$ to compute $x=log(t)+u$.
The only question is whether $x$ will end up in your range from $0$ to $1$.
Unfortunately it never does. I don't have a slick theoretical proof of this, but by numerical experiment, as $t$ increases from $1$ to $e$ (after which point there is clearly no hope because the $log t$ term alone will be too large), the required $x$ seems to increase monotonically from $1$ to about $1.22$.
(For $tge 1$ solving for $u$ gives $u=-W(-t/e^t)$, so we can get Wolfram Alpha to plot $x$ as a function of $t$).
So there is no solution with $tne u$ given your constraints on $x$ -- but for every $x>1$ there is one.
$endgroup$
$begingroup$
Hmm, this seems to be rather complete now. I'll try to apply this problem of when $x gt 1$ to the basic question which I've linked to.
$endgroup$
– Gottfried Helms
Dec 24 '18 at 1:37
$begingroup$
Next idea: I get that $u=W(exp(x+a))$ and $t=W(exp(x-a))$ . It has to be shown that $a=0$ is required when $x in 0..1$ . Perhaps this can make it smoother, but yet I do not see how....
$endgroup$
– Gottfried Helms
Dec 24 '18 at 8:28
add a comment |
$begingroup$
I've got possibly an ansatz, but get stuck at the end.
$$ begin{array} {rl|rl|l}
t &= exp(x-u) & u &= exp(x-t) & implies \
t exp(u) &= exp(x) & u exp(t) &= exp(x) & \
end{array}
$$ so we have
$$ t exp(u) = u exp(t) qquad gt 0 $$
See a contourplot for $log(t)+u = log(u)+t$ drawn by W/A:
Assume now, that $u=t+d$ then we get
$$ t exp(t) exp(d) = (t+d)exp(t) \
t exp(d) = t+d \
t (1+d+d^2/2+ cdots) = t+d qquad text{ we know $t gt 0$ so we can cancel}\
(1+d+d^2/2+ cdots) = 1+d/t \
d+d^2/2+ cdots = d/t \
$$
Hmm. If $t ge 1$ we have our contradiction and $d=0$ is required. But if $t lt d$ I'm again stuck....
$endgroup$
add a comment |
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2 Answers
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$begingroup$
Rearranging, you get:
$$ tag{1} te^u = e^x = ue^t $$
and from there
$$ tag{2} frac{t}{e^t} = frac{u}{e^u} $$
The function $tmapsto t/e^t$ increases from $t=0$ to $t=1$, but after that it starts falling again -- so you can find $tne u$ that satisfies $(2)$ quite fine, and from there use $(1)$ to compute $x=log(t)+u$.
The only question is whether $x$ will end up in your range from $0$ to $1$.
Unfortunately it never does. I don't have a slick theoretical proof of this, but by numerical experiment, as $t$ increases from $1$ to $e$ (after which point there is clearly no hope because the $log t$ term alone will be too large), the required $x$ seems to increase monotonically from $1$ to about $1.22$.
(For $tge 1$ solving for $u$ gives $u=-W(-t/e^t)$, so we can get Wolfram Alpha to plot $x$ as a function of $t$).
So there is no solution with $tne u$ given your constraints on $x$ -- but for every $x>1$ there is one.
$endgroup$
$begingroup$
Hmm, this seems to be rather complete now. I'll try to apply this problem of when $x gt 1$ to the basic question which I've linked to.
$endgroup$
– Gottfried Helms
Dec 24 '18 at 1:37
$begingroup$
Next idea: I get that $u=W(exp(x+a))$ and $t=W(exp(x-a))$ . It has to be shown that $a=0$ is required when $x in 0..1$ . Perhaps this can make it smoother, but yet I do not see how....
$endgroup$
– Gottfried Helms
Dec 24 '18 at 8:28
add a comment |
$begingroup$
Rearranging, you get:
$$ tag{1} te^u = e^x = ue^t $$
and from there
$$ tag{2} frac{t}{e^t} = frac{u}{e^u} $$
The function $tmapsto t/e^t$ increases from $t=0$ to $t=1$, but after that it starts falling again -- so you can find $tne u$ that satisfies $(2)$ quite fine, and from there use $(1)$ to compute $x=log(t)+u$.
The only question is whether $x$ will end up in your range from $0$ to $1$.
Unfortunately it never does. I don't have a slick theoretical proof of this, but by numerical experiment, as $t$ increases from $1$ to $e$ (after which point there is clearly no hope because the $log t$ term alone will be too large), the required $x$ seems to increase monotonically from $1$ to about $1.22$.
(For $tge 1$ solving for $u$ gives $u=-W(-t/e^t)$, so we can get Wolfram Alpha to plot $x$ as a function of $t$).
So there is no solution with $tne u$ given your constraints on $x$ -- but for every $x>1$ there is one.
$endgroup$
$begingroup$
Hmm, this seems to be rather complete now. I'll try to apply this problem of when $x gt 1$ to the basic question which I've linked to.
$endgroup$
– Gottfried Helms
Dec 24 '18 at 1:37
$begingroup$
Next idea: I get that $u=W(exp(x+a))$ and $t=W(exp(x-a))$ . It has to be shown that $a=0$ is required when $x in 0..1$ . Perhaps this can make it smoother, but yet I do not see how....
$endgroup$
– Gottfried Helms
Dec 24 '18 at 8:28
add a comment |
$begingroup$
Rearranging, you get:
$$ tag{1} te^u = e^x = ue^t $$
and from there
$$ tag{2} frac{t}{e^t} = frac{u}{e^u} $$
The function $tmapsto t/e^t$ increases from $t=0$ to $t=1$, but after that it starts falling again -- so you can find $tne u$ that satisfies $(2)$ quite fine, and from there use $(1)$ to compute $x=log(t)+u$.
The only question is whether $x$ will end up in your range from $0$ to $1$.
Unfortunately it never does. I don't have a slick theoretical proof of this, but by numerical experiment, as $t$ increases from $1$ to $e$ (after which point there is clearly no hope because the $log t$ term alone will be too large), the required $x$ seems to increase monotonically from $1$ to about $1.22$.
(For $tge 1$ solving for $u$ gives $u=-W(-t/e^t)$, so we can get Wolfram Alpha to plot $x$ as a function of $t$).
So there is no solution with $tne u$ given your constraints on $x$ -- but for every $x>1$ there is one.
$endgroup$
Rearranging, you get:
$$ tag{1} te^u = e^x = ue^t $$
and from there
$$ tag{2} frac{t}{e^t} = frac{u}{e^u} $$
The function $tmapsto t/e^t$ increases from $t=0$ to $t=1$, but after that it starts falling again -- so you can find $tne u$ that satisfies $(2)$ quite fine, and from there use $(1)$ to compute $x=log(t)+u$.
The only question is whether $x$ will end up in your range from $0$ to $1$.
Unfortunately it never does. I don't have a slick theoretical proof of this, but by numerical experiment, as $t$ increases from $1$ to $e$ (after which point there is clearly no hope because the $log t$ term alone will be too large), the required $x$ seems to increase monotonically from $1$ to about $1.22$.
(For $tge 1$ solving for $u$ gives $u=-W(-t/e^t)$, so we can get Wolfram Alpha to plot $x$ as a function of $t$).
So there is no solution with $tne u$ given your constraints on $x$ -- but for every $x>1$ there is one.
edited Dec 24 '18 at 1:29
answered Dec 24 '18 at 0:59
Henning MakholmHenning Makholm
240k17305541
240k17305541
$begingroup$
Hmm, this seems to be rather complete now. I'll try to apply this problem of when $x gt 1$ to the basic question which I've linked to.
$endgroup$
– Gottfried Helms
Dec 24 '18 at 1:37
$begingroup$
Next idea: I get that $u=W(exp(x+a))$ and $t=W(exp(x-a))$ . It has to be shown that $a=0$ is required when $x in 0..1$ . Perhaps this can make it smoother, but yet I do not see how....
$endgroup$
– Gottfried Helms
Dec 24 '18 at 8:28
add a comment |
$begingroup$
Hmm, this seems to be rather complete now. I'll try to apply this problem of when $x gt 1$ to the basic question which I've linked to.
$endgroup$
– Gottfried Helms
Dec 24 '18 at 1:37
$begingroup$
Next idea: I get that $u=W(exp(x+a))$ and $t=W(exp(x-a))$ . It has to be shown that $a=0$ is required when $x in 0..1$ . Perhaps this can make it smoother, but yet I do not see how....
$endgroup$
– Gottfried Helms
Dec 24 '18 at 8:28
$begingroup$
Hmm, this seems to be rather complete now. I'll try to apply this problem of when $x gt 1$ to the basic question which I've linked to.
$endgroup$
– Gottfried Helms
Dec 24 '18 at 1:37
$begingroup$
Hmm, this seems to be rather complete now. I'll try to apply this problem of when $x gt 1$ to the basic question which I've linked to.
$endgroup$
– Gottfried Helms
Dec 24 '18 at 1:37
$begingroup$
Next idea: I get that $u=W(exp(x+a))$ and $t=W(exp(x-a))$ . It has to be shown that $a=0$ is required when $x in 0..1$ . Perhaps this can make it smoother, but yet I do not see how....
$endgroup$
– Gottfried Helms
Dec 24 '18 at 8:28
$begingroup$
Next idea: I get that $u=W(exp(x+a))$ and $t=W(exp(x-a))$ . It has to be shown that $a=0$ is required when $x in 0..1$ . Perhaps this can make it smoother, but yet I do not see how....
$endgroup$
– Gottfried Helms
Dec 24 '18 at 8:28
add a comment |
$begingroup$
I've got possibly an ansatz, but get stuck at the end.
$$ begin{array} {rl|rl|l}
t &= exp(x-u) & u &= exp(x-t) & implies \
t exp(u) &= exp(x) & u exp(t) &= exp(x) & \
end{array}
$$ so we have
$$ t exp(u) = u exp(t) qquad gt 0 $$
See a contourplot for $log(t)+u = log(u)+t$ drawn by W/A:
Assume now, that $u=t+d$ then we get
$$ t exp(t) exp(d) = (t+d)exp(t) \
t exp(d) = t+d \
t (1+d+d^2/2+ cdots) = t+d qquad text{ we know $t gt 0$ so we can cancel}\
(1+d+d^2/2+ cdots) = 1+d/t \
d+d^2/2+ cdots = d/t \
$$
Hmm. If $t ge 1$ we have our contradiction and $d=0$ is required. But if $t lt d$ I'm again stuck....
$endgroup$
add a comment |
$begingroup$
I've got possibly an ansatz, but get stuck at the end.
$$ begin{array} {rl|rl|l}
t &= exp(x-u) & u &= exp(x-t) & implies \
t exp(u) &= exp(x) & u exp(t) &= exp(x) & \
end{array}
$$ so we have
$$ t exp(u) = u exp(t) qquad gt 0 $$
See a contourplot for $log(t)+u = log(u)+t$ drawn by W/A:
Assume now, that $u=t+d$ then we get
$$ t exp(t) exp(d) = (t+d)exp(t) \
t exp(d) = t+d \
t (1+d+d^2/2+ cdots) = t+d qquad text{ we know $t gt 0$ so we can cancel}\
(1+d+d^2/2+ cdots) = 1+d/t \
d+d^2/2+ cdots = d/t \
$$
Hmm. If $t ge 1$ we have our contradiction and $d=0$ is required. But if $t lt d$ I'm again stuck....
$endgroup$
add a comment |
$begingroup$
I've got possibly an ansatz, but get stuck at the end.
$$ begin{array} {rl|rl|l}
t &= exp(x-u) & u &= exp(x-t) & implies \
t exp(u) &= exp(x) & u exp(t) &= exp(x) & \
end{array}
$$ so we have
$$ t exp(u) = u exp(t) qquad gt 0 $$
See a contourplot for $log(t)+u = log(u)+t$ drawn by W/A:
Assume now, that $u=t+d$ then we get
$$ t exp(t) exp(d) = (t+d)exp(t) \
t exp(d) = t+d \
t (1+d+d^2/2+ cdots) = t+d qquad text{ we know $t gt 0$ so we can cancel}\
(1+d+d^2/2+ cdots) = 1+d/t \
d+d^2/2+ cdots = d/t \
$$
Hmm. If $t ge 1$ we have our contradiction and $d=0$ is required. But if $t lt d$ I'm again stuck....
$endgroup$
I've got possibly an ansatz, but get stuck at the end.
$$ begin{array} {rl|rl|l}
t &= exp(x-u) & u &= exp(x-t) & implies \
t exp(u) &= exp(x) & u exp(t) &= exp(x) & \
end{array}
$$ so we have
$$ t exp(u) = u exp(t) qquad gt 0 $$
See a contourplot for $log(t)+u = log(u)+t$ drawn by W/A:
Assume now, that $u=t+d$ then we get
$$ t exp(t) exp(d) = (t+d)exp(t) \
t exp(d) = t+d \
t (1+d+d^2/2+ cdots) = t+d qquad text{ we know $t gt 0$ so we can cancel}\
(1+d+d^2/2+ cdots) = 1+d/t \
d+d^2/2+ cdots = d/t \
$$
Hmm. If $t ge 1$ we have our contradiction and $d=0$ is required. But if $t lt d$ I'm again stuck....
edited Dec 24 '18 at 2:33
answered Dec 24 '18 at 1:05
Gottfried HelmsGottfried Helms
23.4k24598
23.4k24598
add a comment |
add a comment |
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$begingroup$
@coffeemath, you have the right idea but, unless I misread the question, you should be using the reciprocal of the function you specify.
$endgroup$
– John Omielan
Dec 24 '18 at 0:16
$begingroup$
@JohnOmielan But $f$ is 1-1 iff $1/f$ is.
$endgroup$
– coffeemath
Dec 24 '18 at 0:17
1
$begingroup$
@coffeemath Thanks for explaining. You are right. I did not think this is what you were doing.
$endgroup$
– John Omielan
Dec 24 '18 at 0:19