# Something is going on with the primes…

The chart below illustrates the Erdős–Kac theorem. This relates to the number of distinct prime factors of large numbers (integer sequence A001221 in the On-Line Encyclopedia):

Number No. of prime factors No. of distinct prime factors
1 0 0
2 (prime) 1 1
3 (prime) 1 1
4 = 2×2 2 1
5 (prime) 1 1
6 = 2×3 2 2
7 (prime) 1 1
8 = 2×2×2 3 1
9 = 3×3 2 1
10 = 2×5 2 2
11 (prime) 1 1
12 = 2×2×3 3 2
13 (prime) 1 1
14 = 2×7 2 2
15 = 3×5 2 2
16 = 2×2×2×2 4 1

The Erdős–Kac theorem says that, for large numbers n, the number of distinct prime factors of numbers near n approaches a normal distribution with mean and variance log(log(n)), where the logarithms are to the base e. That seems to be saying that prime numbers are (in some sense) randomly distributed, which is very odd indeed.

In the chart, the observed mean of 3.32 is close to log(log(109)) = 3.03, although the observed variance of 1.36 is smaller. The sample in the chart includes 17 numbers with 8 distinct factors, including 1,000,081,530 = 2×3×3×5×7×19×29×43×67 (9 factors, 8 of which are distinct).

The Erdős–Kac theorem led to an episode where, following the death of Paul Erdős in 1996, Carl Pomerance spoke about the theorem at a conference session in honour of Erdős in 1997. Quoting Albert Einstein (“God does not play dice with the universe”), Pomerance went on to say that he would like to think that Erdős and [Mark] Kac replied “Maybe so, but something is going on with the primes.” The quote is now widely misattributed to Erdős himself.

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