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Prime Number Theorem


The theorem giving an asymptotic form for the Prime Counting Function $\pi(n)$ for number of Primes less than some Integer $n$. Legendre (1808) suggested that, for large $n$,

\pi(n)\sim{n\over A\ln n+B},
\end{displaymath} (1)

with $A=1$ and $B=-1.08366$ (where $B$ is sometimes called Legendre's Constant), a formula which is correct in the leading term only (Wagon 1991, pp. 28-29). In 1791, Gauß became the first to suggest instead
\pi(n)\sim {n\over\ln n}.
\end{displaymath} (2)

Gauß later refined his estimate to
\pi(n)\sim\mathop{\rm Li}\nolimits (n),
\end{displaymath} (3)

where $\mathop{\rm Li}\nolimits (n)$ is the Logarithmic Integral. This function has $n/\ln n$ as the leading term and has been shown to be a better estimate than $n/\ln n$ alone. The statement (3) is often known as ``the'' prime number theorem and was proved independently by Hadamard and Vallée Poussin in 1896. A plot of $\pi(n)$ (lower curve) and $\mathop{\rm Li}\nolimits (n)$ is shown above for $n\leq 1000$.

For small $n$, it has been checked and always found that $\pi(n)<\mathop{\rm Li}\nolimits (n)$. However, Skewes proved that the first crossing of $\pi(n)-\mathop{\rm Li}\nolimits (n)=0$ occurs before $10^{10^{10^{34}}}$ (the Skewes Number). The upper bound for the crossing has subsequently been reduced to $10^{371}$. Littlewood (1914) proved that the Inequality reverses infinitely often for sufficiently large $n$ (Ball and Coxeter 1987). Lehman (1966) proved that at least $10^{500}$ reversals occur for numbers with 1166 or 1167 Decimal Digits.

Chebyshev (Rubinstein and Sarnak 1994) put limits on the Ratio

{7\over 8} < {\pi(n)\over {n\over\ln n}} < {9\over 8},
\end{displaymath} (4)

and showed that if the Limit
\lim_{n\to\infty} {\pi(n)\over {n\over \ln n}}
\end{displaymath} (5)

existed, then it would be 1. This is, in fact, the prime number theorem.

Hadamard and Vallée Poussin proved the prime number theorem by showing that the Riemann Zeta Function $\zeta(z)$ has no zeros of the form $1+it$ (Smith 1994, p. 128). In particular, Vallée Poussin showed that

\pi(x)=\mathop{\rm Li}\nolimits (x)+{\mathcal O}\left({{x\over \ln x} e^{-a\sqrt{\ln x}}\,}\right)
\end{displaymath} (6)

for some constant $a$. A simplified proof was found by Selberg and Erdös (1949) (Ball and Coxeter 1987, p. 63).

Riemann estimated the Prime Counting Function with

\pi(n)\sim \mathop{\rm Li}\nolimits (n)-{\textstyle{1\over 2}}\mathop{\rm Li}\nolimits (n^{1/2}),
\end{displaymath} (7)

which is a better approximation than $\mathop{\rm Li}\nolimits (n)$ for $n<10^7$. Riemann (1859) also suggested the Riemann Function
R(x)=\sum_{n=1}^\infty {\mu(n)\over n} \mathop{\rm Li}\nolimits (x^{1/n}),
\end{displaymath} (8)

where $\mu$ is the Möbius Function (Wagon 1991, p. 29). An even better approximation for small $n$ (by a factor of 10 for $n<10^9$) is the Gram Series.

The prime number theorem is equivalent to

\lim_{x\to\infty} {\psi(x)\over x} = 1,
\end{displaymath} (9)

where $\psi(x)$ is the Summatory Mangoldt Function.

The Riemann Hypothesis is equivalent to the assertion that

\vert\mathop{\rm Li}\nolimits (x)-\pi(x)\vert\leq c\sqrt{x}\,\ln x
\end{displaymath} (10)

for some value of $c$ (Ingham 1932, Ball and Coxeter 1987). Some limits obtained without assuming the Riemann Hypothesis are
$\displaystyle \pi(x)$ $\textstyle =$ $\displaystyle \mathop{\rm Li}\nolimits (x)+{\mathcal O}[xe^{-(\ln x)^{1/2}/15}]$ (11)
$\displaystyle \pi(x)$ $\textstyle =$ $\displaystyle \mathop{\rm Li}\nolimits (x)+{\mathcal O}[xe^{-0.009(\ln x)^{3/5}/(\ln \ln x)^{1/5}}].$ (12)

Ramanujan showed that for sufficiently large $x$,

\pi^2(x)<{ex\over \ln x}\pi\left({x\over e}\right).
\end{displaymath} (13)

The largest known Prime for which the inequality fails is 38,358,837,677 (Berndt 1994, pp. 112-113). The related inequality
\mathop{\rm Li}\nolimits ^2(x)<{ex\over\ln x}\mathop{\rm Li}\nolimits \left({x\over e}\right)
\end{displaymath} (14)

is true for $x\geq 2418$ (Berndt 1994, p. 114).

See also Bertrand's Postulate, Dirichlet's Theorem, Gram Series, Prime Counting Function, Riemann's Formula, Riemann Function, Riemann-Mangoldt Function, Riemann Weighted Prime-Power Counting Function, Skewes Number


Ball, W. W. R. and Coxeter, H. S. M. Mathematical Recreations and Essays, 13th ed. New York: Dover, pp. 62-64, 1987.

Berndt, B. C. Ramanujan's Notebooks, Part IV. New York: Springer-Verlag, 1994.

Courant, R. and Robbins, H. ``The Prime Number Theorem.'' §1.2c in Supplement to Ch. 1 in What is Mathematics?: An Elementary Approach to Ideas and Methods, 2nd ed. Oxford, England: Oxford University Press, pp. 27-30, 1996.

Davenport, H. ``Prime Number Theorem.'' Ch. 18 in Multiplicative Number Theory, 2nd ed. New York: Springer-Verlag, pp. 111-114, 1980.

de la Vallée Poussin, C.-J. ``Recherches analytiques la théorie des nombres premiers.'' Ann. Soc. scient. Bruxelles 20, 183-256, 1896.

Hadamard, J. ``Sur la distribution des zéros de la fonction $\zeta(s)$ et ses conséquences arithmétiques (').'' Bull. Soc. math. France 24, 199-220, 1896.

Hardy, G. H. and Wright, E. M. ``Statement of the Prime Number Theorem.'' §1.8 in An Introduction to the Theory of Numbers, 5th ed. Oxford, England: Clarendon Press, pp. 9-10, 1979.

Ingham, A. E. The Distribution of Prime Numbers. London: Cambridge University Press, p. 83, 1932.

Legendre, A. M. Essai sur la Théorie des Nombres. Paris: Duprat, 1808.

Lehman, R. S. ``On the Difference $\pi(x)-\mathop{\rm li}(x)$.'' Acta Arith. 11, 397-410, 1966.

Littlewood, J. E. ``Sur les distribution des nombres premiers.'' C. R. Acad. Sci. Paris 158, 1869-1872, 1914.

Nagell, T. ``The Prime Number Theorem.'' Ch. 8 in Introduction to Number Theory. New York: Wiley, 1951.

Riemann, G. F. B. ``Über die Anzahl der Primzahlen unter einer gegebenen Grösse.'' Monatsber. Königl. Preuss. Akad. Wiss. Berlin, 671, 1859.

Rubinstein, M. and Sarnak, P. ``Chebyshev's Bias.'' Experimental Math. 3, 173-197, 1994.

Selberg, A. and Erdös, P. ``An Elementary Proof of the Prime Number Theorem.'' Ann. Math. 50, 305-313, 1949.

Shanks, D. ``The Prime Number Theorem.'' §1.6 in Solved and Unsolved Problems in Number Theory, 4th ed. New York: Chelsea, pp. 15-17, 1993.

Smith, D. E. A Source Book in Mathematics. New York: Dover, 1994.

Wagon, S. Mathematica in Action. New York: W. H. Freeman, pp. 25-35, 1991.

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