How do you prove that integer powers of $1.5$ are not equal to any integer powers of $2$?
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I've read somewhere that no integer power of $1.5$ can ever equal any integer power of $2$ (besides the zeroth power, of course). It makes sense, but how is this proven?
(The application here is music. Octaves have a frequency ratio of $2:1$. Pure $5$ths have a $3:2$ frequency ratio. This issue—octaves never lining up with fifths—leads to either the compromise of temperaments or awful sounding intervals that prevent modulation.)
elementary-number-theory exponentiation
$endgroup$
add a comment |
$begingroup$
I've read somewhere that no integer power of $1.5$ can ever equal any integer power of $2$ (besides the zeroth power, of course). It makes sense, but how is this proven?
(The application here is music. Octaves have a frequency ratio of $2:1$. Pure $5$ths have a $3:2$ frequency ratio. This issue—octaves never lining up with fifths—leads to either the compromise of temperaments or awful sounding intervals that prevent modulation.)
elementary-number-theory exponentiation
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2
$begingroup$
You probably mean integer powers?
$endgroup$
– Martin R
Feb 2 at 14:19
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Yes, I do indeed.
$endgroup$
– trw
Feb 2 at 14:20
$begingroup$
This isn't a full answer, but you might also be interested in Størmer's theorem, which finds the "closest" you can get two independent musical intervals by raising them to positive integer powers. ("closest" with regards to linear frequency, rather than the more commonly used logarithmic frequency. You can get arbitrarily close by using log frequency since log2(1.5) is irrational)
$endgroup$
– Ryan
Feb 2 at 16:02
add a comment |
$begingroup$
I've read somewhere that no integer power of $1.5$ can ever equal any integer power of $2$ (besides the zeroth power, of course). It makes sense, but how is this proven?
(The application here is music. Octaves have a frequency ratio of $2:1$. Pure $5$ths have a $3:2$ frequency ratio. This issue—octaves never lining up with fifths—leads to either the compromise of temperaments or awful sounding intervals that prevent modulation.)
elementary-number-theory exponentiation
$endgroup$
I've read somewhere that no integer power of $1.5$ can ever equal any integer power of $2$ (besides the zeroth power, of course). It makes sense, but how is this proven?
(The application here is music. Octaves have a frequency ratio of $2:1$. Pure $5$ths have a $3:2$ frequency ratio. This issue—octaves never lining up with fifths—leads to either the compromise of temperaments or awful sounding intervals that prevent modulation.)
elementary-number-theory exponentiation
elementary-number-theory exponentiation
edited Feb 2 at 18:45
Bill Dubuque
212k29195654
212k29195654
asked Feb 2 at 14:18
trwtrw
3281311
3281311
2
$begingroup$
You probably mean integer powers?
$endgroup$
– Martin R
Feb 2 at 14:19
$begingroup$
Yes, I do indeed.
$endgroup$
– trw
Feb 2 at 14:20
$begingroup$
This isn't a full answer, but you might also be interested in Størmer's theorem, which finds the "closest" you can get two independent musical intervals by raising them to positive integer powers. ("closest" with regards to linear frequency, rather than the more commonly used logarithmic frequency. You can get arbitrarily close by using log frequency since log2(1.5) is irrational)
$endgroup$
– Ryan
Feb 2 at 16:02
add a comment |
2
$begingroup$
You probably mean integer powers?
$endgroup$
– Martin R
Feb 2 at 14:19
$begingroup$
Yes, I do indeed.
$endgroup$
– trw
Feb 2 at 14:20
$begingroup$
This isn't a full answer, but you might also be interested in Størmer's theorem, which finds the "closest" you can get two independent musical intervals by raising them to positive integer powers. ("closest" with regards to linear frequency, rather than the more commonly used logarithmic frequency. You can get arbitrarily close by using log frequency since log2(1.5) is irrational)
$endgroup$
– Ryan
Feb 2 at 16:02
2
2
$begingroup$
You probably mean integer powers?
$endgroup$
– Martin R
Feb 2 at 14:19
$begingroup$
You probably mean integer powers?
$endgroup$
– Martin R
Feb 2 at 14:19
$begingroup$
Yes, I do indeed.
$endgroup$
– trw
Feb 2 at 14:20
$begingroup$
Yes, I do indeed.
$endgroup$
– trw
Feb 2 at 14:20
$begingroup$
This isn't a full answer, but you might also be interested in Størmer's theorem, which finds the "closest" you can get two independent musical intervals by raising them to positive integer powers. ("closest" with regards to linear frequency, rather than the more commonly used logarithmic frequency. You can get arbitrarily close by using log frequency since log2(1.5) is irrational)
$endgroup$
– Ryan
Feb 2 at 16:02
$begingroup$
This isn't a full answer, but you might also be interested in Størmer's theorem, which finds the "closest" you can get two independent musical intervals by raising them to positive integer powers. ("closest" with regards to linear frequency, rather than the more commonly used logarithmic frequency. You can get arbitrarily close by using log frequency since log2(1.5) is irrational)
$endgroup$
– Ryan
Feb 2 at 16:02
add a comment |
5 Answers
5
active
oldest
votes
$begingroup$
If $left(frac32right)^n=2^m$, then $3^n=2^{m+n}$. For all but trivial cases, the right hand side is even, whereas the left is always odd.
$endgroup$
$begingroup$
The same can be used to prove for $6^n$ case.
$endgroup$
– hjpotter92
Feb 2 at 15:41
add a comment |
$begingroup$
We have $$1.5^n=left(frac{3}{2}right)^n=frac{3^n}{2^n}$$
For this to equal a power of 2 we need
$$frac{3^n}{2^n}=2^k$$
$$3^n=2^k cdot 2^n = 2^{(n+k)}$$
And as no power of 3 is equal to a power of 2 ($2^a$ and $3^b$ are relatively prime), we cannot have $1.5^a=2^b$.
$endgroup$
add a comment |
$begingroup$
You know that $1.5=frac{3}{2}$.
To have a power of 1.5 equal to a power of 2 means that $$left(dfrac{3}{2}right)^m=2^n$$ for some integers $m,ninmathbb Z$.
This means $2^{m+n}=3^m$. Since $2$ and $3$ are prime numbers, this can't hold unless $m=n=0$ (see the fundamental theorem of number theory).
$endgroup$
add a comment |
$begingroup$
Apart from $1.5^0$, no integer power of $1.5$ is an integer. We have $1.5=frac32$, and the fundamental theorem of arithmetic shows that $frac{3^n}{2^n}$ can never be simplified.
$endgroup$
add a comment |
$begingroup$
If an integer power $,n>0,$ of a fraction $q$ is an integer then $q$ is an integer, by the Rational Root Test.
More generally RRT shows that a fractional root of an integer coefficient polynomial $,f(x)neq 0,$ with $color{#c00}{rm lead coeff= 1},$ must also be an integer, e.g. $,f(x) = color{#c00}{x^n} - a,$ for an integer $,a,,$ is the above case.
Remark $ $ The arguments in the other answers are essentially special cases of the (short and simple) proof of RRT. The above is but a glimpse of the key role that RRT plays in factorization and number theory - which is clarified when one studies more general number rings (e.g. domains with unique prime factorization must be integrally closed, i.e. they must satisfy said $color{#c00}{rm monic}$ case of RRT).
$endgroup$
$begingroup$
@Downvoter If something is not clear then please feel welcome to ask questions and I will be happy to elaborate.
$endgroup$
– Bill Dubuque
Feb 3 at 17:47
add a comment |
Your Answer
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5 Answers
5
active
oldest
votes
5 Answers
5
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
If $left(frac32right)^n=2^m$, then $3^n=2^{m+n}$. For all but trivial cases, the right hand side is even, whereas the left is always odd.
$endgroup$
$begingroup$
The same can be used to prove for $6^n$ case.
$endgroup$
– hjpotter92
Feb 2 at 15:41
add a comment |
$begingroup$
If $left(frac32right)^n=2^m$, then $3^n=2^{m+n}$. For all but trivial cases, the right hand side is even, whereas the left is always odd.
$endgroup$
$begingroup$
The same can be used to prove for $6^n$ case.
$endgroup$
– hjpotter92
Feb 2 at 15:41
add a comment |
$begingroup$
If $left(frac32right)^n=2^m$, then $3^n=2^{m+n}$. For all but trivial cases, the right hand side is even, whereas the left is always odd.
$endgroup$
If $left(frac32right)^n=2^m$, then $3^n=2^{m+n}$. For all but trivial cases, the right hand side is even, whereas the left is always odd.
answered Feb 2 at 14:23
Hagen von EitzenHagen von Eitzen
283k23272507
283k23272507
$begingroup$
The same can be used to prove for $6^n$ case.
$endgroup$
– hjpotter92
Feb 2 at 15:41
add a comment |
$begingroup$
The same can be used to prove for $6^n$ case.
$endgroup$
– hjpotter92
Feb 2 at 15:41
$begingroup$
The same can be used to prove for $6^n$ case.
$endgroup$
– hjpotter92
Feb 2 at 15:41
$begingroup$
The same can be used to prove for $6^n$ case.
$endgroup$
– hjpotter92
Feb 2 at 15:41
add a comment |
$begingroup$
We have $$1.5^n=left(frac{3}{2}right)^n=frac{3^n}{2^n}$$
For this to equal a power of 2 we need
$$frac{3^n}{2^n}=2^k$$
$$3^n=2^k cdot 2^n = 2^{(n+k)}$$
And as no power of 3 is equal to a power of 2 ($2^a$ and $3^b$ are relatively prime), we cannot have $1.5^a=2^b$.
$endgroup$
add a comment |
$begingroup$
We have $$1.5^n=left(frac{3}{2}right)^n=frac{3^n}{2^n}$$
For this to equal a power of 2 we need
$$frac{3^n}{2^n}=2^k$$
$$3^n=2^k cdot 2^n = 2^{(n+k)}$$
And as no power of 3 is equal to a power of 2 ($2^a$ and $3^b$ are relatively prime), we cannot have $1.5^a=2^b$.
$endgroup$
add a comment |
$begingroup$
We have $$1.5^n=left(frac{3}{2}right)^n=frac{3^n}{2^n}$$
For this to equal a power of 2 we need
$$frac{3^n}{2^n}=2^k$$
$$3^n=2^k cdot 2^n = 2^{(n+k)}$$
And as no power of 3 is equal to a power of 2 ($2^a$ and $3^b$ are relatively prime), we cannot have $1.5^a=2^b$.
$endgroup$
We have $$1.5^n=left(frac{3}{2}right)^n=frac{3^n}{2^n}$$
For this to equal a power of 2 we need
$$frac{3^n}{2^n}=2^k$$
$$3^n=2^k cdot 2^n = 2^{(n+k)}$$
And as no power of 3 is equal to a power of 2 ($2^a$ and $3^b$ are relatively prime), we cannot have $1.5^a=2^b$.
edited Feb 2 at 16:14
Mutantoe
623513
623513
answered Feb 2 at 14:23
Peter ForemanPeter Foreman
3,8371216
3,8371216
add a comment |
add a comment |
$begingroup$
You know that $1.5=frac{3}{2}$.
To have a power of 1.5 equal to a power of 2 means that $$left(dfrac{3}{2}right)^m=2^n$$ for some integers $m,ninmathbb Z$.
This means $2^{m+n}=3^m$. Since $2$ and $3$ are prime numbers, this can't hold unless $m=n=0$ (see the fundamental theorem of number theory).
$endgroup$
add a comment |
$begingroup$
You know that $1.5=frac{3}{2}$.
To have a power of 1.5 equal to a power of 2 means that $$left(dfrac{3}{2}right)^m=2^n$$ for some integers $m,ninmathbb Z$.
This means $2^{m+n}=3^m$. Since $2$ and $3$ are prime numbers, this can't hold unless $m=n=0$ (see the fundamental theorem of number theory).
$endgroup$
add a comment |
$begingroup$
You know that $1.5=frac{3}{2}$.
To have a power of 1.5 equal to a power of 2 means that $$left(dfrac{3}{2}right)^m=2^n$$ for some integers $m,ninmathbb Z$.
This means $2^{m+n}=3^m$. Since $2$ and $3$ are prime numbers, this can't hold unless $m=n=0$ (see the fundamental theorem of number theory).
$endgroup$
You know that $1.5=frac{3}{2}$.
To have a power of 1.5 equal to a power of 2 means that $$left(dfrac{3}{2}right)^m=2^n$$ for some integers $m,ninmathbb Z$.
This means $2^{m+n}=3^m$. Since $2$ and $3$ are prime numbers, this can't hold unless $m=n=0$ (see the fundamental theorem of number theory).
answered Feb 2 at 14:22
ScientificaScientifica
6,82941335
6,82941335
add a comment |
add a comment |
$begingroup$
Apart from $1.5^0$, no integer power of $1.5$ is an integer. We have $1.5=frac32$, and the fundamental theorem of arithmetic shows that $frac{3^n}{2^n}$ can never be simplified.
$endgroup$
add a comment |
$begingroup$
Apart from $1.5^0$, no integer power of $1.5$ is an integer. We have $1.5=frac32$, and the fundamental theorem of arithmetic shows that $frac{3^n}{2^n}$ can never be simplified.
$endgroup$
add a comment |
$begingroup$
Apart from $1.5^0$, no integer power of $1.5$ is an integer. We have $1.5=frac32$, and the fundamental theorem of arithmetic shows that $frac{3^n}{2^n}$ can never be simplified.
$endgroup$
Apart from $1.5^0$, no integer power of $1.5$ is an integer. We have $1.5=frac32$, and the fundamental theorem of arithmetic shows that $frac{3^n}{2^n}$ can never be simplified.
answered Feb 2 at 14:21
ArthurArthur
118k7118202
118k7118202
add a comment |
add a comment |
$begingroup$
If an integer power $,n>0,$ of a fraction $q$ is an integer then $q$ is an integer, by the Rational Root Test.
More generally RRT shows that a fractional root of an integer coefficient polynomial $,f(x)neq 0,$ with $color{#c00}{rm lead coeff= 1},$ must also be an integer, e.g. $,f(x) = color{#c00}{x^n} - a,$ for an integer $,a,,$ is the above case.
Remark $ $ The arguments in the other answers are essentially special cases of the (short and simple) proof of RRT. The above is but a glimpse of the key role that RRT plays in factorization and number theory - which is clarified when one studies more general number rings (e.g. domains with unique prime factorization must be integrally closed, i.e. they must satisfy said $color{#c00}{rm monic}$ case of RRT).
$endgroup$
$begingroup$
@Downvoter If something is not clear then please feel welcome to ask questions and I will be happy to elaborate.
$endgroup$
– Bill Dubuque
Feb 3 at 17:47
add a comment |
$begingroup$
If an integer power $,n>0,$ of a fraction $q$ is an integer then $q$ is an integer, by the Rational Root Test.
More generally RRT shows that a fractional root of an integer coefficient polynomial $,f(x)neq 0,$ with $color{#c00}{rm lead coeff= 1},$ must also be an integer, e.g. $,f(x) = color{#c00}{x^n} - a,$ for an integer $,a,,$ is the above case.
Remark $ $ The arguments in the other answers are essentially special cases of the (short and simple) proof of RRT. The above is but a glimpse of the key role that RRT plays in factorization and number theory - which is clarified when one studies more general number rings (e.g. domains with unique prime factorization must be integrally closed, i.e. they must satisfy said $color{#c00}{rm monic}$ case of RRT).
$endgroup$
$begingroup$
@Downvoter If something is not clear then please feel welcome to ask questions and I will be happy to elaborate.
$endgroup$
– Bill Dubuque
Feb 3 at 17:47
add a comment |
$begingroup$
If an integer power $,n>0,$ of a fraction $q$ is an integer then $q$ is an integer, by the Rational Root Test.
More generally RRT shows that a fractional root of an integer coefficient polynomial $,f(x)neq 0,$ with $color{#c00}{rm lead coeff= 1},$ must also be an integer, e.g. $,f(x) = color{#c00}{x^n} - a,$ for an integer $,a,,$ is the above case.
Remark $ $ The arguments in the other answers are essentially special cases of the (short and simple) proof of RRT. The above is but a glimpse of the key role that RRT plays in factorization and number theory - which is clarified when one studies more general number rings (e.g. domains with unique prime factorization must be integrally closed, i.e. they must satisfy said $color{#c00}{rm monic}$ case of RRT).
$endgroup$
If an integer power $,n>0,$ of a fraction $q$ is an integer then $q$ is an integer, by the Rational Root Test.
More generally RRT shows that a fractional root of an integer coefficient polynomial $,f(x)neq 0,$ with $color{#c00}{rm lead coeff= 1},$ must also be an integer, e.g. $,f(x) = color{#c00}{x^n} - a,$ for an integer $,a,,$ is the above case.
Remark $ $ The arguments in the other answers are essentially special cases of the (short and simple) proof of RRT. The above is but a glimpse of the key role that RRT plays in factorization and number theory - which is clarified when one studies more general number rings (e.g. domains with unique prime factorization must be integrally closed, i.e. they must satisfy said $color{#c00}{rm monic}$ case of RRT).
edited Feb 3 at 17:55
answered Feb 2 at 14:50
Bill DubuqueBill Dubuque
212k29195654
212k29195654
$begingroup$
@Downvoter If something is not clear then please feel welcome to ask questions and I will be happy to elaborate.
$endgroup$
– Bill Dubuque
Feb 3 at 17:47
add a comment |
$begingroup$
@Downvoter If something is not clear then please feel welcome to ask questions and I will be happy to elaborate.
$endgroup$
– Bill Dubuque
Feb 3 at 17:47
$begingroup$
@Downvoter If something is not clear then please feel welcome to ask questions and I will be happy to elaborate.
$endgroup$
– Bill Dubuque
Feb 3 at 17:47
$begingroup$
@Downvoter If something is not clear then please feel welcome to ask questions and I will be happy to elaborate.
$endgroup$
– Bill Dubuque
Feb 3 at 17:47
add a comment |
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2
$begingroup$
You probably mean integer powers?
$endgroup$
– Martin R
Feb 2 at 14:19
$begingroup$
Yes, I do indeed.
$endgroup$
– trw
Feb 2 at 14:20
$begingroup$
This isn't a full answer, but you might also be interested in Størmer's theorem, which finds the "closest" you can get two independent musical intervals by raising them to positive integer powers. ("closest" with regards to linear frequency, rather than the more commonly used logarithmic frequency. You can get arbitrarily close by using log frequency since log2(1.5) is irrational)
$endgroup$
– Ryan
Feb 2 at 16:02