Is this a valid definition of the rationals?
$begingroup$
$$mathbb{Q}=left{sum_{n=1}^k f(n)mid k,ninmathbb{N}land ftext{ is a finite composition of $+$, $-$, $div$, $times$}right}$$
Reasoning:
Any real number can be described by a (sometimes infinite) sum of rational numbers. If such a sum is taken to be $q=sum_{n=1}^k f(n)$, then every real number which is not rational can be approximated to arbitrary precision by increasingly large $k$. If $f(n)$ is composed solely of elementary arithmetic operations ($+,-,div,times$), then $q$ remains rational for all $k<infty$.
If the quotient of any two rational numbers is also rational, then it follows that for elementary functions $f$ and $g$, the quotient of the summations
$sum_{n=1}^k f(n)$ and $sum_{n=1}^k g(n)$ is always rational even as $k$ tends towards infinity.
Intuitively, it would seem that the quotient of any two such summations is always rational even if $k=infty$. However, this is not the case, as there are many infinite sums satisfying the above conditions which are irrational.
Therefore, the sum is rational iff the upper bound $k$ is finite.
Becuase any real number may be represented as a summation, it follows that any rational number can be represented as a summation.
Thus, every rational number can be represented as a finite sum of elementary functions.
algebra-precalculus elementary-number-theory summation rational-numbers
$endgroup$
|
show 4 more comments
$begingroup$
$$mathbb{Q}=left{sum_{n=1}^k f(n)mid k,ninmathbb{N}land ftext{ is a finite composition of $+$, $-$, $div$, $times$}right}$$
Reasoning:
Any real number can be described by a (sometimes infinite) sum of rational numbers. If such a sum is taken to be $q=sum_{n=1}^k f(n)$, then every real number which is not rational can be approximated to arbitrary precision by increasingly large $k$. If $f(n)$ is composed solely of elementary arithmetic operations ($+,-,div,times$), then $q$ remains rational for all $k<infty$.
If the quotient of any two rational numbers is also rational, then it follows that for elementary functions $f$ and $g$, the quotient of the summations
$sum_{n=1}^k f(n)$ and $sum_{n=1}^k g(n)$ is always rational even as $k$ tends towards infinity.
Intuitively, it would seem that the quotient of any two such summations is always rational even if $k=infty$. However, this is not the case, as there are many infinite sums satisfying the above conditions which are irrational.
Therefore, the sum is rational iff the upper bound $k$ is finite.
Becuase any real number may be represented as a summation, it follows that any rational number can be represented as a summation.
Thus, every rational number can be represented as a finite sum of elementary functions.
algebra-precalculus elementary-number-theory summation rational-numbers
$endgroup$
2
$begingroup$
Note that $sum_{n=1}^k$ is itself a finite composition of $+$, so it is redundant. It's enough with just $f(1)$.
$endgroup$
– Arthur
Dec 18 '18 at 16:40
2
$begingroup$
How do you define "a finite composition of +, −, ÷, ×"?
$endgroup$
– BPP
Dec 18 '18 at 16:42
$begingroup$
@Arthur it is redundant if $k$ is finite, yes. The only reason it was important to specify was because of the 'natural' extension to the reals - that being the case where $k$ is infinite.
$endgroup$
– R. Burton
Dec 18 '18 at 16:44
2
$begingroup$
"Therefore, the sum is rational iff the upper bound k is finite." Nope - consider $sum_{i=1}^infty 2^{-i}$.
$endgroup$
– Noah Schweber
Dec 18 '18 at 16:50
3
$begingroup$
This whole post seems like it's a more muddy construction of the rationals that requires a lot of casework and cleanup. I'm not sure I see the purpose.
$endgroup$
– Don Thousand
Dec 18 '18 at 16:53
|
show 4 more comments
$begingroup$
$$mathbb{Q}=left{sum_{n=1}^k f(n)mid k,ninmathbb{N}land ftext{ is a finite composition of $+$, $-$, $div$, $times$}right}$$
Reasoning:
Any real number can be described by a (sometimes infinite) sum of rational numbers. If such a sum is taken to be $q=sum_{n=1}^k f(n)$, then every real number which is not rational can be approximated to arbitrary precision by increasingly large $k$. If $f(n)$ is composed solely of elementary arithmetic operations ($+,-,div,times$), then $q$ remains rational for all $k<infty$.
If the quotient of any two rational numbers is also rational, then it follows that for elementary functions $f$ and $g$, the quotient of the summations
$sum_{n=1}^k f(n)$ and $sum_{n=1}^k g(n)$ is always rational even as $k$ tends towards infinity.
Intuitively, it would seem that the quotient of any two such summations is always rational even if $k=infty$. However, this is not the case, as there are many infinite sums satisfying the above conditions which are irrational.
Therefore, the sum is rational iff the upper bound $k$ is finite.
Becuase any real number may be represented as a summation, it follows that any rational number can be represented as a summation.
Thus, every rational number can be represented as a finite sum of elementary functions.
algebra-precalculus elementary-number-theory summation rational-numbers
$endgroup$
$$mathbb{Q}=left{sum_{n=1}^k f(n)mid k,ninmathbb{N}land ftext{ is a finite composition of $+$, $-$, $div$, $times$}right}$$
Reasoning:
Any real number can be described by a (sometimes infinite) sum of rational numbers. If such a sum is taken to be $q=sum_{n=1}^k f(n)$, then every real number which is not rational can be approximated to arbitrary precision by increasingly large $k$. If $f(n)$ is composed solely of elementary arithmetic operations ($+,-,div,times$), then $q$ remains rational for all $k<infty$.
If the quotient of any two rational numbers is also rational, then it follows that for elementary functions $f$ and $g$, the quotient of the summations
$sum_{n=1}^k f(n)$ and $sum_{n=1}^k g(n)$ is always rational even as $k$ tends towards infinity.
Intuitively, it would seem that the quotient of any two such summations is always rational even if $k=infty$. However, this is not the case, as there are many infinite sums satisfying the above conditions which are irrational.
Therefore, the sum is rational iff the upper bound $k$ is finite.
Becuase any real number may be represented as a summation, it follows that any rational number can be represented as a summation.
Thus, every rational number can be represented as a finite sum of elementary functions.
algebra-precalculus elementary-number-theory summation rational-numbers
algebra-precalculus elementary-number-theory summation rational-numbers
asked Dec 18 '18 at 16:39
R. BurtonR. Burton
687110
687110
2
$begingroup$
Note that $sum_{n=1}^k$ is itself a finite composition of $+$, so it is redundant. It's enough with just $f(1)$.
$endgroup$
– Arthur
Dec 18 '18 at 16:40
2
$begingroup$
How do you define "a finite composition of +, −, ÷, ×"?
$endgroup$
– BPP
Dec 18 '18 at 16:42
$begingroup$
@Arthur it is redundant if $k$ is finite, yes. The only reason it was important to specify was because of the 'natural' extension to the reals - that being the case where $k$ is infinite.
$endgroup$
– R. Burton
Dec 18 '18 at 16:44
2
$begingroup$
"Therefore, the sum is rational iff the upper bound k is finite." Nope - consider $sum_{i=1}^infty 2^{-i}$.
$endgroup$
– Noah Schweber
Dec 18 '18 at 16:50
3
$begingroup$
This whole post seems like it's a more muddy construction of the rationals that requires a lot of casework and cleanup. I'm not sure I see the purpose.
$endgroup$
– Don Thousand
Dec 18 '18 at 16:53
|
show 4 more comments
2
$begingroup$
Note that $sum_{n=1}^k$ is itself a finite composition of $+$, so it is redundant. It's enough with just $f(1)$.
$endgroup$
– Arthur
Dec 18 '18 at 16:40
2
$begingroup$
How do you define "a finite composition of +, −, ÷, ×"?
$endgroup$
– BPP
Dec 18 '18 at 16:42
$begingroup$
@Arthur it is redundant if $k$ is finite, yes. The only reason it was important to specify was because of the 'natural' extension to the reals - that being the case where $k$ is infinite.
$endgroup$
– R. Burton
Dec 18 '18 at 16:44
2
$begingroup$
"Therefore, the sum is rational iff the upper bound k is finite." Nope - consider $sum_{i=1}^infty 2^{-i}$.
$endgroup$
– Noah Schweber
Dec 18 '18 at 16:50
3
$begingroup$
This whole post seems like it's a more muddy construction of the rationals that requires a lot of casework and cleanup. I'm not sure I see the purpose.
$endgroup$
– Don Thousand
Dec 18 '18 at 16:53
2
2
$begingroup$
Note that $sum_{n=1}^k$ is itself a finite composition of $+$, so it is redundant. It's enough with just $f(1)$.
$endgroup$
– Arthur
Dec 18 '18 at 16:40
$begingroup$
Note that $sum_{n=1}^k$ is itself a finite composition of $+$, so it is redundant. It's enough with just $f(1)$.
$endgroup$
– Arthur
Dec 18 '18 at 16:40
2
2
$begingroup$
How do you define "a finite composition of +, −, ÷, ×"?
$endgroup$
– BPP
Dec 18 '18 at 16:42
$begingroup$
How do you define "a finite composition of +, −, ÷, ×"?
$endgroup$
– BPP
Dec 18 '18 at 16:42
$begingroup$
@Arthur it is redundant if $k$ is finite, yes. The only reason it was important to specify was because of the 'natural' extension to the reals - that being the case where $k$ is infinite.
$endgroup$
– R. Burton
Dec 18 '18 at 16:44
$begingroup$
@Arthur it is redundant if $k$ is finite, yes. The only reason it was important to specify was because of the 'natural' extension to the reals - that being the case where $k$ is infinite.
$endgroup$
– R. Burton
Dec 18 '18 at 16:44
2
2
$begingroup$
"Therefore, the sum is rational iff the upper bound k is finite." Nope - consider $sum_{i=1}^infty 2^{-i}$.
$endgroup$
– Noah Schweber
Dec 18 '18 at 16:50
$begingroup$
"Therefore, the sum is rational iff the upper bound k is finite." Nope - consider $sum_{i=1}^infty 2^{-i}$.
$endgroup$
– Noah Schweber
Dec 18 '18 at 16:50
3
3
$begingroup$
This whole post seems like it's a more muddy construction of the rationals that requires a lot of casework and cleanup. I'm not sure I see the purpose.
$endgroup$
– Don Thousand
Dec 18 '18 at 16:53
$begingroup$
This whole post seems like it's a more muddy construction of the rationals that requires a lot of casework and cleanup. I'm not sure I see the purpose.
$endgroup$
– Don Thousand
Dec 18 '18 at 16:53
|
show 4 more comments
1 Answer
1
active
oldest
votes
$begingroup$
If $k$ is finite, then as noted above there's no need to use the summation notation at all. The right thing to say in my opinion is:
$mathbb{Q}$ is the smallest set of reals containing $1$ and closed under $+,times,-,$ and $div$.
One direction is easy to prove: A positive rational ${aover b}$ can always be represented as $(1+...+1)over (1+...+1)$ with $a$-many $1$s in the numerator and $b$-many $1$s in the denominator. $0$ can be gotten as $1-1$, and this lets us take negatives. So $mathbb{Q}$ is contained in any such set. Conversely, clearly $1inmathbb{Q}$ and $mathbb{Q}$ is closed under $+,times,-,$ and $div$, so we're done.
Meanwhile, every real can be represented as an infinite sum of rationals, and so allowing the sum to be infinite does indeed get all of $mathbb{R}$. However, there are two caveats worth noting. First, not every infinite sum corresponds to a real (an infinite sum can diverge or oscillate). Second, some infinite sums do still correspond to rationals, contra your claim "the sum is rational iff the upper bound $k$ is finite." For example, consider $$sum_{i=1}^infty{1div 2times ...times 2mbox{ ($i$ times)}},$$ more clearly written as $sum_{i=1}^infty 2^{-i}$, which is of course just $1$.
$endgroup$
3
$begingroup$
I want to emphasize that the wording suggested here is cleaner than what was posted in the question, which is not rigorous and difficult to parse. Better to use words and a careful description (as in this answer) than a lot of symbols.
$endgroup$
– Andrés E. Caicedo
Dec 18 '18 at 16:58
add a comment |
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$begingroup$
If $k$ is finite, then as noted above there's no need to use the summation notation at all. The right thing to say in my opinion is:
$mathbb{Q}$ is the smallest set of reals containing $1$ and closed under $+,times,-,$ and $div$.
One direction is easy to prove: A positive rational ${aover b}$ can always be represented as $(1+...+1)over (1+...+1)$ with $a$-many $1$s in the numerator and $b$-many $1$s in the denominator. $0$ can be gotten as $1-1$, and this lets us take negatives. So $mathbb{Q}$ is contained in any such set. Conversely, clearly $1inmathbb{Q}$ and $mathbb{Q}$ is closed under $+,times,-,$ and $div$, so we're done.
Meanwhile, every real can be represented as an infinite sum of rationals, and so allowing the sum to be infinite does indeed get all of $mathbb{R}$. However, there are two caveats worth noting. First, not every infinite sum corresponds to a real (an infinite sum can diverge or oscillate). Second, some infinite sums do still correspond to rationals, contra your claim "the sum is rational iff the upper bound $k$ is finite." For example, consider $$sum_{i=1}^infty{1div 2times ...times 2mbox{ ($i$ times)}},$$ more clearly written as $sum_{i=1}^infty 2^{-i}$, which is of course just $1$.
$endgroup$
3
$begingroup$
I want to emphasize that the wording suggested here is cleaner than what was posted in the question, which is not rigorous and difficult to parse. Better to use words and a careful description (as in this answer) than a lot of symbols.
$endgroup$
– Andrés E. Caicedo
Dec 18 '18 at 16:58
add a comment |
$begingroup$
If $k$ is finite, then as noted above there's no need to use the summation notation at all. The right thing to say in my opinion is:
$mathbb{Q}$ is the smallest set of reals containing $1$ and closed under $+,times,-,$ and $div$.
One direction is easy to prove: A positive rational ${aover b}$ can always be represented as $(1+...+1)over (1+...+1)$ with $a$-many $1$s in the numerator and $b$-many $1$s in the denominator. $0$ can be gotten as $1-1$, and this lets us take negatives. So $mathbb{Q}$ is contained in any such set. Conversely, clearly $1inmathbb{Q}$ and $mathbb{Q}$ is closed under $+,times,-,$ and $div$, so we're done.
Meanwhile, every real can be represented as an infinite sum of rationals, and so allowing the sum to be infinite does indeed get all of $mathbb{R}$. However, there are two caveats worth noting. First, not every infinite sum corresponds to a real (an infinite sum can diverge or oscillate). Second, some infinite sums do still correspond to rationals, contra your claim "the sum is rational iff the upper bound $k$ is finite." For example, consider $$sum_{i=1}^infty{1div 2times ...times 2mbox{ ($i$ times)}},$$ more clearly written as $sum_{i=1}^infty 2^{-i}$, which is of course just $1$.
$endgroup$
3
$begingroup$
I want to emphasize that the wording suggested here is cleaner than what was posted in the question, which is not rigorous and difficult to parse. Better to use words and a careful description (as in this answer) than a lot of symbols.
$endgroup$
– Andrés E. Caicedo
Dec 18 '18 at 16:58
add a comment |
$begingroup$
If $k$ is finite, then as noted above there's no need to use the summation notation at all. The right thing to say in my opinion is:
$mathbb{Q}$ is the smallest set of reals containing $1$ and closed under $+,times,-,$ and $div$.
One direction is easy to prove: A positive rational ${aover b}$ can always be represented as $(1+...+1)over (1+...+1)$ with $a$-many $1$s in the numerator and $b$-many $1$s in the denominator. $0$ can be gotten as $1-1$, and this lets us take negatives. So $mathbb{Q}$ is contained in any such set. Conversely, clearly $1inmathbb{Q}$ and $mathbb{Q}$ is closed under $+,times,-,$ and $div$, so we're done.
Meanwhile, every real can be represented as an infinite sum of rationals, and so allowing the sum to be infinite does indeed get all of $mathbb{R}$. However, there are two caveats worth noting. First, not every infinite sum corresponds to a real (an infinite sum can diverge or oscillate). Second, some infinite sums do still correspond to rationals, contra your claim "the sum is rational iff the upper bound $k$ is finite." For example, consider $$sum_{i=1}^infty{1div 2times ...times 2mbox{ ($i$ times)}},$$ more clearly written as $sum_{i=1}^infty 2^{-i}$, which is of course just $1$.
$endgroup$
If $k$ is finite, then as noted above there's no need to use the summation notation at all. The right thing to say in my opinion is:
$mathbb{Q}$ is the smallest set of reals containing $1$ and closed under $+,times,-,$ and $div$.
One direction is easy to prove: A positive rational ${aover b}$ can always be represented as $(1+...+1)over (1+...+1)$ with $a$-many $1$s in the numerator and $b$-many $1$s in the denominator. $0$ can be gotten as $1-1$, and this lets us take negatives. So $mathbb{Q}$ is contained in any such set. Conversely, clearly $1inmathbb{Q}$ and $mathbb{Q}$ is closed under $+,times,-,$ and $div$, so we're done.
Meanwhile, every real can be represented as an infinite sum of rationals, and so allowing the sum to be infinite does indeed get all of $mathbb{R}$. However, there are two caveats worth noting. First, not every infinite sum corresponds to a real (an infinite sum can diverge or oscillate). Second, some infinite sums do still correspond to rationals, contra your claim "the sum is rational iff the upper bound $k$ is finite." For example, consider $$sum_{i=1}^infty{1div 2times ...times 2mbox{ ($i$ times)}},$$ more clearly written as $sum_{i=1}^infty 2^{-i}$, which is of course just $1$.
answered Dec 18 '18 at 16:55
Noah SchweberNoah Schweber
127k10151293
127k10151293
3
$begingroup$
I want to emphasize that the wording suggested here is cleaner than what was posted in the question, which is not rigorous and difficult to parse. Better to use words and a careful description (as in this answer) than a lot of symbols.
$endgroup$
– Andrés E. Caicedo
Dec 18 '18 at 16:58
add a comment |
3
$begingroup$
I want to emphasize that the wording suggested here is cleaner than what was posted in the question, which is not rigorous and difficult to parse. Better to use words and a careful description (as in this answer) than a lot of symbols.
$endgroup$
– Andrés E. Caicedo
Dec 18 '18 at 16:58
3
3
$begingroup$
I want to emphasize that the wording suggested here is cleaner than what was posted in the question, which is not rigorous and difficult to parse. Better to use words and a careful description (as in this answer) than a lot of symbols.
$endgroup$
– Andrés E. Caicedo
Dec 18 '18 at 16:58
$begingroup$
I want to emphasize that the wording suggested here is cleaner than what was posted in the question, which is not rigorous and difficult to parse. Better to use words and a careful description (as in this answer) than a lot of symbols.
$endgroup$
– Andrés E. Caicedo
Dec 18 '18 at 16:58
add a comment |
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2
$begingroup$
Note that $sum_{n=1}^k$ is itself a finite composition of $+$, so it is redundant. It's enough with just $f(1)$.
$endgroup$
– Arthur
Dec 18 '18 at 16:40
2
$begingroup$
How do you define "a finite composition of +, −, ÷, ×"?
$endgroup$
– BPP
Dec 18 '18 at 16:42
$begingroup$
@Arthur it is redundant if $k$ is finite, yes. The only reason it was important to specify was because of the 'natural' extension to the reals - that being the case where $k$ is infinite.
$endgroup$
– R. Burton
Dec 18 '18 at 16:44
2
$begingroup$
"Therefore, the sum is rational iff the upper bound k is finite." Nope - consider $sum_{i=1}^infty 2^{-i}$.
$endgroup$
– Noah Schweber
Dec 18 '18 at 16:50
3
$begingroup$
This whole post seems like it's a more muddy construction of the rationals that requires a lot of casework and cleanup. I'm not sure I see the purpose.
$endgroup$
– Don Thousand
Dec 18 '18 at 16:53