Asymptotic expression for the $n$th prime number











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Quoting from the Wikipedia article:




As a consequence of the prime number theorem, one gets an asymptotic expression for the $ n $th prime number, denoted by $ p_n $:
$$ p_n sim n log n.$$




Can you explain how we get this approximate expression? I understand the number of prime numbers less than or equal to an integer $ N $, denoted as $ pi(N) $ is approximately:
$$
pi(N) sim frac{N}{log N}.
$$



If the $ n $ prime number is approximately $ n log n $, it implies that there are approximately $ n $ prime numbers less than or equal to $ n log n $, i.e.,
$$
pi(n log n) sim n.
$$

But substituting $ N $ with $ n log n $ in the formula of $pi(N)$ we get
$$
pi(N) sim frac{N}{log N} = frac{n log n}{log (n log n)} notsim n.
$$

What am I doing wrong? How can I show that the statement quoted from Wikipedia is correct?










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  • log(n log n) is very close to log(n).
    – Jalex Stark
    May 9 at 3:24















up vote
2
down vote

favorite
1












Quoting from the Wikipedia article:




As a consequence of the prime number theorem, one gets an asymptotic expression for the $ n $th prime number, denoted by $ p_n $:
$$ p_n sim n log n.$$




Can you explain how we get this approximate expression? I understand the number of prime numbers less than or equal to an integer $ N $, denoted as $ pi(N) $ is approximately:
$$
pi(N) sim frac{N}{log N}.
$$



If the $ n $ prime number is approximately $ n log n $, it implies that there are approximately $ n $ prime numbers less than or equal to $ n log n $, i.e.,
$$
pi(n log n) sim n.
$$

But substituting $ N $ with $ n log n $ in the formula of $pi(N)$ we get
$$
pi(N) sim frac{N}{log N} = frac{n log n}{log (n log n)} notsim n.
$$

What am I doing wrong? How can I show that the statement quoted from Wikipedia is correct?










share|cite|improve this question
























  • log(n log n) is very close to log(n).
    – Jalex Stark
    May 9 at 3:24













up vote
2
down vote

favorite
1









up vote
2
down vote

favorite
1






1





Quoting from the Wikipedia article:




As a consequence of the prime number theorem, one gets an asymptotic expression for the $ n $th prime number, denoted by $ p_n $:
$$ p_n sim n log n.$$




Can you explain how we get this approximate expression? I understand the number of prime numbers less than or equal to an integer $ N $, denoted as $ pi(N) $ is approximately:
$$
pi(N) sim frac{N}{log N}.
$$



If the $ n $ prime number is approximately $ n log n $, it implies that there are approximately $ n $ prime numbers less than or equal to $ n log n $, i.e.,
$$
pi(n log n) sim n.
$$

But substituting $ N $ with $ n log n $ in the formula of $pi(N)$ we get
$$
pi(N) sim frac{N}{log N} = frac{n log n}{log (n log n)} notsim n.
$$

What am I doing wrong? How can I show that the statement quoted from Wikipedia is correct?










share|cite|improve this question















Quoting from the Wikipedia article:




As a consequence of the prime number theorem, one gets an asymptotic expression for the $ n $th prime number, denoted by $ p_n $:
$$ p_n sim n log n.$$




Can you explain how we get this approximate expression? I understand the number of prime numbers less than or equal to an integer $ N $, denoted as $ pi(N) $ is approximately:
$$
pi(N) sim frac{N}{log N}.
$$



If the $ n $ prime number is approximately $ n log n $, it implies that there are approximately $ n $ prime numbers less than or equal to $ n log n $, i.e.,
$$
pi(n log n) sim n.
$$

But substituting $ N $ with $ n log n $ in the formula of $pi(N)$ we get
$$
pi(N) sim frac{N}{log N} = frac{n log n}{log (n log n)} notsim n.
$$

What am I doing wrong? How can I show that the statement quoted from Wikipedia is correct?







prime-numbers






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edited Nov 23 at 13:55









Klangen

1,49011232




1,49011232










asked May 9 at 3:17









Lone Learner

249416




249416












  • log(n log n) is very close to log(n).
    – Jalex Stark
    May 9 at 3:24


















  • log(n log n) is very close to log(n).
    – Jalex Stark
    May 9 at 3:24
















log(n log n) is very close to log(n).
– Jalex Stark
May 9 at 3:24




log(n log n) is very close to log(n).
– Jalex Stark
May 9 at 3:24










2 Answers
2






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oldest

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1
down vote













$$log(nlog n)=log n+loglog n=(1+o(1))log n$$
therefore
$$frac{nlog n}{log(nlog n)}=frac{nlog n}{(1+o(1))log n}
=frac{n}{(1+o(1))}sim n.$$






share|cite|improve this answer




























    up vote
    0
    down vote













    It seems you are very new to prime number theory hence I am not sure if you understand the little $o$ notations. Hence I present @Lord Shark's answer in layman's terms.



    $$
    log (nlog n) = log n + loglog n
    $$



    But $loglog n$ is exponentially smaller compared to $log n$ i.e. it can be ignore compared to $log n$. Hence as an approximation, we have



    $$
    log (nlog n) sim log n
    $$



    With this approximation



    $$
    pi (nlog n) sim frac{nlog n}{log (nlog n)} sim frac{nlog n}{log n} sim n
    $$



    The smallest values of $x$ for which $pi(x) = n$ is $x = p_n$ hence we have



    $$
    pi (p_n) = n sim pi(nlog n)
    $$



    or $p_n sim nlog n$.






    share|cite|improve this answer





















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      2 Answers
      2






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      2 Answers
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      up vote
      1
      down vote













      $$log(nlog n)=log n+loglog n=(1+o(1))log n$$
      therefore
      $$frac{nlog n}{log(nlog n)}=frac{nlog n}{(1+o(1))log n}
      =frac{n}{(1+o(1))}sim n.$$






      share|cite|improve this answer

























        up vote
        1
        down vote













        $$log(nlog n)=log n+loglog n=(1+o(1))log n$$
        therefore
        $$frac{nlog n}{log(nlog n)}=frac{nlog n}{(1+o(1))log n}
        =frac{n}{(1+o(1))}sim n.$$






        share|cite|improve this answer























          up vote
          1
          down vote










          up vote
          1
          down vote









          $$log(nlog n)=log n+loglog n=(1+o(1))log n$$
          therefore
          $$frac{nlog n}{log(nlog n)}=frac{nlog n}{(1+o(1))log n}
          =frac{n}{(1+o(1))}sim n.$$






          share|cite|improve this answer












          $$log(nlog n)=log n+loglog n=(1+o(1))log n$$
          therefore
          $$frac{nlog n}{log(nlog n)}=frac{nlog n}{(1+o(1))log n}
          =frac{n}{(1+o(1))}sim n.$$







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered May 9 at 3:25









          Lord Shark the Unknown

          99.6k958131




          99.6k958131






















              up vote
              0
              down vote













              It seems you are very new to prime number theory hence I am not sure if you understand the little $o$ notations. Hence I present @Lord Shark's answer in layman's terms.



              $$
              log (nlog n) = log n + loglog n
              $$



              But $loglog n$ is exponentially smaller compared to $log n$ i.e. it can be ignore compared to $log n$. Hence as an approximation, we have



              $$
              log (nlog n) sim log n
              $$



              With this approximation



              $$
              pi (nlog n) sim frac{nlog n}{log (nlog n)} sim frac{nlog n}{log n} sim n
              $$



              The smallest values of $x$ for which $pi(x) = n$ is $x = p_n$ hence we have



              $$
              pi (p_n) = n sim pi(nlog n)
              $$



              or $p_n sim nlog n$.






              share|cite|improve this answer

























                up vote
                0
                down vote













                It seems you are very new to prime number theory hence I am not sure if you understand the little $o$ notations. Hence I present @Lord Shark's answer in layman's terms.



                $$
                log (nlog n) = log n + loglog n
                $$



                But $loglog n$ is exponentially smaller compared to $log n$ i.e. it can be ignore compared to $log n$. Hence as an approximation, we have



                $$
                log (nlog n) sim log n
                $$



                With this approximation



                $$
                pi (nlog n) sim frac{nlog n}{log (nlog n)} sim frac{nlog n}{log n} sim n
                $$



                The smallest values of $x$ for which $pi(x) = n$ is $x = p_n$ hence we have



                $$
                pi (p_n) = n sim pi(nlog n)
                $$



                or $p_n sim nlog n$.






                share|cite|improve this answer























                  up vote
                  0
                  down vote










                  up vote
                  0
                  down vote









                  It seems you are very new to prime number theory hence I am not sure if you understand the little $o$ notations. Hence I present @Lord Shark's answer in layman's terms.



                  $$
                  log (nlog n) = log n + loglog n
                  $$



                  But $loglog n$ is exponentially smaller compared to $log n$ i.e. it can be ignore compared to $log n$. Hence as an approximation, we have



                  $$
                  log (nlog n) sim log n
                  $$



                  With this approximation



                  $$
                  pi (nlog n) sim frac{nlog n}{log (nlog n)} sim frac{nlog n}{log n} sim n
                  $$



                  The smallest values of $x$ for which $pi(x) = n$ is $x = p_n$ hence we have



                  $$
                  pi (p_n) = n sim pi(nlog n)
                  $$



                  or $p_n sim nlog n$.






                  share|cite|improve this answer












                  It seems you are very new to prime number theory hence I am not sure if you understand the little $o$ notations. Hence I present @Lord Shark's answer in layman's terms.



                  $$
                  log (nlog n) = log n + loglog n
                  $$



                  But $loglog n$ is exponentially smaller compared to $log n$ i.e. it can be ignore compared to $log n$. Hence as an approximation, we have



                  $$
                  log (nlog n) sim log n
                  $$



                  With this approximation



                  $$
                  pi (nlog n) sim frac{nlog n}{log (nlog n)} sim frac{nlog n}{log n} sim n
                  $$



                  The smallest values of $x$ for which $pi(x) = n$ is $x = p_n$ hence we have



                  $$
                  pi (p_n) = n sim pi(nlog n)
                  $$



                  or $p_n sim nlog n$.







                  share|cite|improve this answer












                  share|cite|improve this answer



                  share|cite|improve this answer










                  answered May 9 at 4:18









                  Nilotpal Kanti Sinha

                  4,01721437




                  4,01721437






























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