Is this a valid definition of the rationals?












0












$begingroup$


$$mathbb{Q}=left{sum_{n=1}^k f(n)mid k,ninmathbb{N}land ftext{ is a finite composition of $+$, $-$, $div$, $times$}right}$$



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.










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$endgroup$








  • 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
















0












$begingroup$


$$mathbb{Q}=left{sum_{n=1}^k f(n)mid k,ninmathbb{N}land ftext{ is a finite composition of $+$, $-$, $div$, $times$}right}$$



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.










share|cite|improve this question









$endgroup$








  • 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














0












0








0





$begingroup$


$$mathbb{Q}=left{sum_{n=1}^k f(n)mid k,ninmathbb{N}land ftext{ is a finite composition of $+$, $-$, $div$, $times$}right}$$



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.










share|cite|improve this question









$endgroup$




$$mathbb{Q}=left{sum_{n=1}^k f(n)mid k,ninmathbb{N}land ftext{ is a finite composition of $+$, $-$, $div$, $times$}right}$$



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






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










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














  • 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










1 Answer
1






active

oldest

votes


















4












$begingroup$

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 $1$.






share|cite|improve this answer









$endgroup$









  • 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












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1 Answer
1






active

oldest

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1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









4












$begingroup$

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 $1$.






share|cite|improve this answer









$endgroup$









  • 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
















4












$begingroup$

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 $1$.






share|cite|improve this answer









$endgroup$









  • 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














4












4








4





$begingroup$

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 $1$.






share|cite|improve this answer









$endgroup$



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 $1$.







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










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














  • 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


















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