Chapter 4: Systems Based on Numbers

Section 4: The Sequence of Primes

Results about primes

Prime[n] is given approximately by n Log[n] + n Log[Log[n]]. (Prime[109] is 22,801,763,489 while the approximation gives 2.38 × 1010.) A first approximation to PrimePi[n] is n/Log[n]. A somewhat better approximation is LogIntegral[n], equal to Integrate[1/Log[t], {t, 2, n}]. This was found empirically by Carl Friedrich Gauss in 1792, based on looking at a table of primes. (PrimePi[109] is 50,847,534 while LogIntegral[109] is about 50,849,235.) A still better approximation is obtained by subtracting Sum[LogIntegral[nri], {i, -, }] where the ri are the complex zeros of the Riemann zeta function Zeta[s], discussed on page 918. According to the Riemann Hypothesis, the difference between PrimePi[n] and LogIntegral[n] is of order n Log[n]. More refined analytical estimates of PrimePi[n] are good enough that they are used by Mathematica to compute Prime[n] for large n.

It is known that the ratio of the number of primes of the form 4k + 1 and 4k + 3 asymptotically approaches 1, but almost nothing has been proved about the fluctuations.

The gap between successive primes Prime[n] - Prime[n - 1] is thought to grow on average at most like Log[Prime[n]]2. It is known that for sufficiently large n a gap of any size must exist. It is believed but not proved that there are an infinite number of "twin primes" with a gap of exactly 2.

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From Stephen Wolfram: A New Kind of Science [citation]