Proof of Bertrand's postulate












2












$begingroup$


The following proof is from the 19th page of
Everest, Graham; Ward, Thomas, An introduction to number theory, Graduate Texts in Mathematics 232. London: Springer (ISBN 1-85233-917-9/hbk). x, 294 p. (2005). ZBL1089.11001.



In fact, I think this proof is not finished. For the red line, only $k(p)ge 2$ is disproved. For the case $k(p)=1$, there is not any talk. How to understand it ? Thanks.




$textbf{Theorem 1.9.} [text{B}{scriptstyle{text{ERTRAND'S}}} text{ P}scriptstyle{text{OSTULATE}}] $ If $ngeqslant1$, then there is at least one prime $p$ with the property that $$n<pleqslant2n.tag{1.13}$$ $text{P}scriptstyle{text{ROOF}}$. For any real number $x$, let $lfloor xrfloor$ denote the integer part of $x$. Thus $lfloor xrfloor$ is the greatest integer less than or equal to $x$. Let $p$ be any prime. Then $$leftlfloordfrac nprightrfloor+leftlfloordfrac n{p^2}rightrfloor+leftlfloordfrac n{p^3}rightrfloor+cdots$$ is the largest power of $p$ dividing $n!$ (see Exercise $8.7(a)$ on p. $162$). Fix $ngeqslant 1$ and let $$N=prod_{pleqslant2n}p^{k(p)}$$ be the prime decomposition of $N=(2n)!/(n!)^2$. The number of times that a given prime $p$ divides $N$ is the difference between the number of times it divides $(2n)!$ and $(n!)^2$, so $$k(p)=sum_{m=1}^inftyleft(leftlfloordfrac{2n}{p^m}rightrfloor-2leftlfloordfrac n{p^m}rightrfloorright),tag{1.14}$$ and each of the terms in the sum is either $0$ or $1$, depending on whether $leftlfloorfrac{2n}{p^m}rightrfloor$ is odd or even. If $p^m>2n$ the term is certainly $0$, so $$k(p)leqslantleftlfloordfrac{log2n}{log p}rightrfloor.tag{1.15}$$



enter image description here



enter image description here











share|cite|improve this question











$endgroup$












  • $begingroup$
    Already the definition of $k(p)$ is confusing.
    $endgroup$
    – Peter
    Jan 9 at 10:06










  • $begingroup$
    @Peter I think the definition of $k(p)$ is very ok
    $endgroup$
    – lanse7pty
    Jan 9 at 10:11










  • $begingroup$
    But hard to actually understand. We should find out what $k(p)=1$ means. Perhaps this case is trivial for the desired proof.
    $endgroup$
    – Peter
    Jan 9 at 10:13










  • $begingroup$
    $k(p)$ is the exponent of the prime $p$ in the prime decomposition of $N$. $k(p)$ is well-defined, unique and greater or equal to $0$.
    $endgroup$
    – Snake707
    Jan 9 at 10:30










  • $begingroup$
    @Snake707 I did not claim that $k(p)$ is not well defined.
    $endgroup$
    – Peter
    Jan 9 at 10:57
















2












$begingroup$


The following proof is from the 19th page of
Everest, Graham; Ward, Thomas, An introduction to number theory, Graduate Texts in Mathematics 232. London: Springer (ISBN 1-85233-917-9/hbk). x, 294 p. (2005). ZBL1089.11001.



In fact, I think this proof is not finished. For the red line, only $k(p)ge 2$ is disproved. For the case $k(p)=1$, there is not any talk. How to understand it ? Thanks.




$textbf{Theorem 1.9.} [text{B}{scriptstyle{text{ERTRAND'S}}} text{ P}scriptstyle{text{OSTULATE}}] $ If $ngeqslant1$, then there is at least one prime $p$ with the property that $$n<pleqslant2n.tag{1.13}$$ $text{P}scriptstyle{text{ROOF}}$. For any real number $x$, let $lfloor xrfloor$ denote the integer part of $x$. Thus $lfloor xrfloor$ is the greatest integer less than or equal to $x$. Let $p$ be any prime. Then $$leftlfloordfrac nprightrfloor+leftlfloordfrac n{p^2}rightrfloor+leftlfloordfrac n{p^3}rightrfloor+cdots$$ is the largest power of $p$ dividing $n!$ (see Exercise $8.7(a)$ on p. $162$). Fix $ngeqslant 1$ and let $$N=prod_{pleqslant2n}p^{k(p)}$$ be the prime decomposition of $N=(2n)!/(n!)^2$. The number of times that a given prime $p$ divides $N$ is the difference between the number of times it divides $(2n)!$ and $(n!)^2$, so $$k(p)=sum_{m=1}^inftyleft(leftlfloordfrac{2n}{p^m}rightrfloor-2leftlfloordfrac n{p^m}rightrfloorright),tag{1.14}$$ and each of the terms in the sum is either $0$ or $1$, depending on whether $leftlfloorfrac{2n}{p^m}rightrfloor$ is odd or even. If $p^m>2n$ the term is certainly $0$, so $$k(p)leqslantleftlfloordfrac{log2n}{log p}rightrfloor.tag{1.15}$$



enter image description here



enter image description here











share|cite|improve this question











$endgroup$












  • $begingroup$
    Already the definition of $k(p)$ is confusing.
    $endgroup$
    – Peter
    Jan 9 at 10:06










  • $begingroup$
    @Peter I think the definition of $k(p)$ is very ok
    $endgroup$
    – lanse7pty
    Jan 9 at 10:11










  • $begingroup$
    But hard to actually understand. We should find out what $k(p)=1$ means. Perhaps this case is trivial for the desired proof.
    $endgroup$
    – Peter
    Jan 9 at 10:13










  • $begingroup$
    $k(p)$ is the exponent of the prime $p$ in the prime decomposition of $N$. $k(p)$ is well-defined, unique and greater or equal to $0$.
    $endgroup$
    – Snake707
    Jan 9 at 10:30










  • $begingroup$
    @Snake707 I did not claim that $k(p)$ is not well defined.
    $endgroup$
    – Peter
    Jan 9 at 10:57














2












2








2


1



$begingroup$


The following proof is from the 19th page of
Everest, Graham; Ward, Thomas, An introduction to number theory, Graduate Texts in Mathematics 232. London: Springer (ISBN 1-85233-917-9/hbk). x, 294 p. (2005). ZBL1089.11001.



In fact, I think this proof is not finished. For the red line, only $k(p)ge 2$ is disproved. For the case $k(p)=1$, there is not any talk. How to understand it ? Thanks.




$textbf{Theorem 1.9.} [text{B}{scriptstyle{text{ERTRAND'S}}} text{ P}scriptstyle{text{OSTULATE}}] $ If $ngeqslant1$, then there is at least one prime $p$ with the property that $$n<pleqslant2n.tag{1.13}$$ $text{P}scriptstyle{text{ROOF}}$. For any real number $x$, let $lfloor xrfloor$ denote the integer part of $x$. Thus $lfloor xrfloor$ is the greatest integer less than or equal to $x$. Let $p$ be any prime. Then $$leftlfloordfrac nprightrfloor+leftlfloordfrac n{p^2}rightrfloor+leftlfloordfrac n{p^3}rightrfloor+cdots$$ is the largest power of $p$ dividing $n!$ (see Exercise $8.7(a)$ on p. $162$). Fix $ngeqslant 1$ and let $$N=prod_{pleqslant2n}p^{k(p)}$$ be the prime decomposition of $N=(2n)!/(n!)^2$. The number of times that a given prime $p$ divides $N$ is the difference between the number of times it divides $(2n)!$ and $(n!)^2$, so $$k(p)=sum_{m=1}^inftyleft(leftlfloordfrac{2n}{p^m}rightrfloor-2leftlfloordfrac n{p^m}rightrfloorright),tag{1.14}$$ and each of the terms in the sum is either $0$ or $1$, depending on whether $leftlfloorfrac{2n}{p^m}rightrfloor$ is odd or even. If $p^m>2n$ the term is certainly $0$, so $$k(p)leqslantleftlfloordfrac{log2n}{log p}rightrfloor.tag{1.15}$$



enter image description here



enter image description here











share|cite|improve this question











$endgroup$




The following proof is from the 19th page of
Everest, Graham; Ward, Thomas, An introduction to number theory, Graduate Texts in Mathematics 232. London: Springer (ISBN 1-85233-917-9/hbk). x, 294 p. (2005). ZBL1089.11001.



In fact, I think this proof is not finished. For the red line, only $k(p)ge 2$ is disproved. For the case $k(p)=1$, there is not any talk. How to understand it ? Thanks.




$textbf{Theorem 1.9.} [text{B}{scriptstyle{text{ERTRAND'S}}} text{ P}scriptstyle{text{OSTULATE}}] $ If $ngeqslant1$, then there is at least one prime $p$ with the property that $$n<pleqslant2n.tag{1.13}$$ $text{P}scriptstyle{text{ROOF}}$. For any real number $x$, let $lfloor xrfloor$ denote the integer part of $x$. Thus $lfloor xrfloor$ is the greatest integer less than or equal to $x$. Let $p$ be any prime. Then $$leftlfloordfrac nprightrfloor+leftlfloordfrac n{p^2}rightrfloor+leftlfloordfrac n{p^3}rightrfloor+cdots$$ is the largest power of $p$ dividing $n!$ (see Exercise $8.7(a)$ on p. $162$). Fix $ngeqslant 1$ and let $$N=prod_{pleqslant2n}p^{k(p)}$$ be the prime decomposition of $N=(2n)!/(n!)^2$. The number of times that a given prime $p$ divides $N$ is the difference between the number of times it divides $(2n)!$ and $(n!)^2$, so $$k(p)=sum_{m=1}^inftyleft(leftlfloordfrac{2n}{p^m}rightrfloor-2leftlfloordfrac n{p^m}rightrfloorright),tag{1.14}$$ and each of the terms in the sum is either $0$ or $1$, depending on whether $leftlfloorfrac{2n}{p^m}rightrfloor$ is odd or even. If $p^m>2n$ the term is certainly $0$, so $$k(p)leqslantleftlfloordfrac{log2n}{log p}rightrfloor.tag{1.15}$$



enter image description here



enter image description here








number-theory






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share|cite|improve this question













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share|cite|improve this question








edited Jan 27 at 9:58









Citation Needed

31




31










asked Jan 9 at 9:58









lanse7ptylanse7pty

1,7811823




1,7811823












  • $begingroup$
    Already the definition of $k(p)$ is confusing.
    $endgroup$
    – Peter
    Jan 9 at 10:06










  • $begingroup$
    @Peter I think the definition of $k(p)$ is very ok
    $endgroup$
    – lanse7pty
    Jan 9 at 10:11










  • $begingroup$
    But hard to actually understand. We should find out what $k(p)=1$ means. Perhaps this case is trivial for the desired proof.
    $endgroup$
    – Peter
    Jan 9 at 10:13










  • $begingroup$
    $k(p)$ is the exponent of the prime $p$ in the prime decomposition of $N$. $k(p)$ is well-defined, unique and greater or equal to $0$.
    $endgroup$
    – Snake707
    Jan 9 at 10:30










  • $begingroup$
    @Snake707 I did not claim that $k(p)$ is not well defined.
    $endgroup$
    – Peter
    Jan 9 at 10:57


















  • $begingroup$
    Already the definition of $k(p)$ is confusing.
    $endgroup$
    – Peter
    Jan 9 at 10:06










  • $begingroup$
    @Peter I think the definition of $k(p)$ is very ok
    $endgroup$
    – lanse7pty
    Jan 9 at 10:11










  • $begingroup$
    But hard to actually understand. We should find out what $k(p)=1$ means. Perhaps this case is trivial for the desired proof.
    $endgroup$
    – Peter
    Jan 9 at 10:13










  • $begingroup$
    $k(p)$ is the exponent of the prime $p$ in the prime decomposition of $N$. $k(p)$ is well-defined, unique and greater or equal to $0$.
    $endgroup$
    – Snake707
    Jan 9 at 10:30










  • $begingroup$
    @Snake707 I did not claim that $k(p)$ is not well defined.
    $endgroup$
    – Peter
    Jan 9 at 10:57
















$begingroup$
Already the definition of $k(p)$ is confusing.
$endgroup$
– Peter
Jan 9 at 10:06




$begingroup$
Already the definition of $k(p)$ is confusing.
$endgroup$
– Peter
Jan 9 at 10:06












$begingroup$
@Peter I think the definition of $k(p)$ is very ok
$endgroup$
– lanse7pty
Jan 9 at 10:11




$begingroup$
@Peter I think the definition of $k(p)$ is very ok
$endgroup$
– lanse7pty
Jan 9 at 10:11












$begingroup$
But hard to actually understand. We should find out what $k(p)=1$ means. Perhaps this case is trivial for the desired proof.
$endgroup$
– Peter
Jan 9 at 10:13




$begingroup$
But hard to actually understand. We should find out what $k(p)=1$ means. Perhaps this case is trivial for the desired proof.
$endgroup$
– Peter
Jan 9 at 10:13












$begingroup$
$k(p)$ is the exponent of the prime $p$ in the prime decomposition of $N$. $k(p)$ is well-defined, unique and greater or equal to $0$.
$endgroup$
– Snake707
Jan 9 at 10:30




$begingroup$
$k(p)$ is the exponent of the prime $p$ in the prime decomposition of $N$. $k(p)$ is well-defined, unique and greater or equal to $0$.
$endgroup$
– Snake707
Jan 9 at 10:30












$begingroup$
@Snake707 I did not claim that $k(p)$ is not well defined.
$endgroup$
– Peter
Jan 9 at 10:57




$begingroup$
@Snake707 I did not claim that $k(p)$ is not well defined.
$endgroup$
– Peter
Jan 9 at 10:57










1 Answer
1






active

oldest

votes


















1












$begingroup$

The idea is to notice that



$$log(N) leq sum_{p|N}{log(p)} + sum_{k(p) geq 2}{k(p)log(p)}.$$



The first term is dealt with by $(1.16)$, the second one by the last estimate of the image before last.






share|cite|improve this answer









$endgroup$









  • 1




    $begingroup$
    Could you talk it detail ? I can't understand you. Thanks.
    $endgroup$
    – lanse7pty
    Jan 9 at 11:44










  • $begingroup$
    Have you read your page $21$?
    $endgroup$
    – Mindlack
    Jan 9 at 11:45












  • $begingroup$
    Yes, I have read it, but seemly, there is not anything about the case $k(p)=1$.
    $endgroup$
    – lanse7pty
    Jan 9 at 11:58










  • $begingroup$
    In the derivation of $(1.17)$, you count every prime once, and then you count all primes with $k(p) geq 2$ $k(p)$ times. So primes with $k(p)=1$ are counted once and primes with $k(p) > 1$ are counted $k(p)+1$ times.
    $endgroup$
    – Mindlack
    Jan 9 at 12:08










  • $begingroup$
    I understand you, thanks.
    $endgroup$
    – lanse7pty
    Jan 9 at 12:46











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1 Answer
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1 Answer
1






active

oldest

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active

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active

oldest

votes









1












$begingroup$

The idea is to notice that



$$log(N) leq sum_{p|N}{log(p)} + sum_{k(p) geq 2}{k(p)log(p)}.$$



The first term is dealt with by $(1.16)$, the second one by the last estimate of the image before last.






share|cite|improve this answer









$endgroup$









  • 1




    $begingroup$
    Could you talk it detail ? I can't understand you. Thanks.
    $endgroup$
    – lanse7pty
    Jan 9 at 11:44










  • $begingroup$
    Have you read your page $21$?
    $endgroup$
    – Mindlack
    Jan 9 at 11:45












  • $begingroup$
    Yes, I have read it, but seemly, there is not anything about the case $k(p)=1$.
    $endgroup$
    – lanse7pty
    Jan 9 at 11:58










  • $begingroup$
    In the derivation of $(1.17)$, you count every prime once, and then you count all primes with $k(p) geq 2$ $k(p)$ times. So primes with $k(p)=1$ are counted once and primes with $k(p) > 1$ are counted $k(p)+1$ times.
    $endgroup$
    – Mindlack
    Jan 9 at 12:08










  • $begingroup$
    I understand you, thanks.
    $endgroup$
    – lanse7pty
    Jan 9 at 12:46
















1












$begingroup$

The idea is to notice that



$$log(N) leq sum_{p|N}{log(p)} + sum_{k(p) geq 2}{k(p)log(p)}.$$



The first term is dealt with by $(1.16)$, the second one by the last estimate of the image before last.






share|cite|improve this answer









$endgroup$









  • 1




    $begingroup$
    Could you talk it detail ? I can't understand you. Thanks.
    $endgroup$
    – lanse7pty
    Jan 9 at 11:44










  • $begingroup$
    Have you read your page $21$?
    $endgroup$
    – Mindlack
    Jan 9 at 11:45












  • $begingroup$
    Yes, I have read it, but seemly, there is not anything about the case $k(p)=1$.
    $endgroup$
    – lanse7pty
    Jan 9 at 11:58










  • $begingroup$
    In the derivation of $(1.17)$, you count every prime once, and then you count all primes with $k(p) geq 2$ $k(p)$ times. So primes with $k(p)=1$ are counted once and primes with $k(p) > 1$ are counted $k(p)+1$ times.
    $endgroup$
    – Mindlack
    Jan 9 at 12:08










  • $begingroup$
    I understand you, thanks.
    $endgroup$
    – lanse7pty
    Jan 9 at 12:46














1












1








1





$begingroup$

The idea is to notice that



$$log(N) leq sum_{p|N}{log(p)} + sum_{k(p) geq 2}{k(p)log(p)}.$$



The first term is dealt with by $(1.16)$, the second one by the last estimate of the image before last.






share|cite|improve this answer









$endgroup$



The idea is to notice that



$$log(N) leq sum_{p|N}{log(p)} + sum_{k(p) geq 2}{k(p)log(p)}.$$



The first term is dealt with by $(1.16)$, the second one by the last estimate of the image before last.







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Jan 9 at 10:30









MindlackMindlack

3,49217




3,49217








  • 1




    $begingroup$
    Could you talk it detail ? I can't understand you. Thanks.
    $endgroup$
    – lanse7pty
    Jan 9 at 11:44










  • $begingroup$
    Have you read your page $21$?
    $endgroup$
    – Mindlack
    Jan 9 at 11:45












  • $begingroup$
    Yes, I have read it, but seemly, there is not anything about the case $k(p)=1$.
    $endgroup$
    – lanse7pty
    Jan 9 at 11:58










  • $begingroup$
    In the derivation of $(1.17)$, you count every prime once, and then you count all primes with $k(p) geq 2$ $k(p)$ times. So primes with $k(p)=1$ are counted once and primes with $k(p) > 1$ are counted $k(p)+1$ times.
    $endgroup$
    – Mindlack
    Jan 9 at 12:08










  • $begingroup$
    I understand you, thanks.
    $endgroup$
    – lanse7pty
    Jan 9 at 12:46














  • 1




    $begingroup$
    Could you talk it detail ? I can't understand you. Thanks.
    $endgroup$
    – lanse7pty
    Jan 9 at 11:44










  • $begingroup$
    Have you read your page $21$?
    $endgroup$
    – Mindlack
    Jan 9 at 11:45












  • $begingroup$
    Yes, I have read it, but seemly, there is not anything about the case $k(p)=1$.
    $endgroup$
    – lanse7pty
    Jan 9 at 11:58










  • $begingroup$
    In the derivation of $(1.17)$, you count every prime once, and then you count all primes with $k(p) geq 2$ $k(p)$ times. So primes with $k(p)=1$ are counted once and primes with $k(p) > 1$ are counted $k(p)+1$ times.
    $endgroup$
    – Mindlack
    Jan 9 at 12:08










  • $begingroup$
    I understand you, thanks.
    $endgroup$
    – lanse7pty
    Jan 9 at 12:46








1




1




$begingroup$
Could you talk it detail ? I can't understand you. Thanks.
$endgroup$
– lanse7pty
Jan 9 at 11:44




$begingroup$
Could you talk it detail ? I can't understand you. Thanks.
$endgroup$
– lanse7pty
Jan 9 at 11:44












$begingroup$
Have you read your page $21$?
$endgroup$
– Mindlack
Jan 9 at 11:45






$begingroup$
Have you read your page $21$?
$endgroup$
– Mindlack
Jan 9 at 11:45














$begingroup$
Yes, I have read it, but seemly, there is not anything about the case $k(p)=1$.
$endgroup$
– lanse7pty
Jan 9 at 11:58




$begingroup$
Yes, I have read it, but seemly, there is not anything about the case $k(p)=1$.
$endgroup$
– lanse7pty
Jan 9 at 11:58












$begingroup$
In the derivation of $(1.17)$, you count every prime once, and then you count all primes with $k(p) geq 2$ $k(p)$ times. So primes with $k(p)=1$ are counted once and primes with $k(p) > 1$ are counted $k(p)+1$ times.
$endgroup$
– Mindlack
Jan 9 at 12:08




$begingroup$
In the derivation of $(1.17)$, you count every prime once, and then you count all primes with $k(p) geq 2$ $k(p)$ times. So primes with $k(p)=1$ are counted once and primes with $k(p) > 1$ are counted $k(p)+1$ times.
$endgroup$
– Mindlack
Jan 9 at 12:08












$begingroup$
I understand you, thanks.
$endgroup$
– lanse7pty
Jan 9 at 12:46




$begingroup$
I understand you, thanks.
$endgroup$
– lanse7pty
Jan 9 at 12:46


















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