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Does the string of prime numbers $$2357111317\ldots$$ contain every natural number as its sub-string?

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It's unlikely to contain π as a substring, because π contains a decimal point and this string doesn't. Do you mean "every integer"? –  gnasher729 Aug 30 at 23:26
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No minus signs, @gnasher729. So has to refer to non-negative integers. :) –  Thomas Andrews Aug 30 at 23:27
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See Copeland-Erdos constant. –  Lucian Aug 30 at 23:30
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@Buddha, you didn't mention natural numbers, so change your question. And extending it to reals would be stupid because your string doesn't contain a decimal point or a minus character, so it doesn't contain the numbers 1.5 and -2, for example. –  gnasher729 Aug 30 at 23:34

2 Answers 2

up vote 52 down vote accepted

It follows from Dirichlet's Theorem.

If $d$ is the number we want to find, define $s=10d+1$. By definition, $\gcd(s,10)=1$ and $s$ contains the digits of $d$.

Dirichlet's Theorem's implies there's a prime of the form $p:=s+k \times 10^n$ where $10^n$ is chosen so that it has as many zeroes as digits of $s$. The digits of $d$ appear in the digits of $p$, and thus in the given string of primes.

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Very nice! This is actually exercise 4.16.b in Apostol's Introduction to Analytic Number Theory. –  Chip Hurst Aug 31 at 5:28
    
You should add that $k\in\mathbb N_{\ge 0}$. –  mathh Aug 31 at 13:26

If you mean "every non-negative integer", then the answer is yes.

First, it contains the integer 0 as a substring because 101 is a prime.

Next, for every integer i > 0, there is a prime that starts with the integer i: Take the known results about gaps between prime numbers, for example that there is always a prime between $n^3$ and$(n+1)^3$ for large n. There is always a large n such that both $n^3$ and$(n+1)^3$ start with the digits of i, therefore the prime between $n^3$ and$(n+1)^3$ also starts with the digits of i.

(This is true because one of i, 10i and 100i is not the cube of an integer).

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I love that you have two solutions here: you show that every $k$ is a prefix of a prime written in the decimal system, the other answer shows that every $k$ is a suffix of it :) –  tohecz Aug 31 at 19:56
    
@tohecz Yes, it actually means there's no need to string the primes together, simply $2,3,5, \dots$ will contain all natural numbers somewhere. –  Mark Hurd Sep 2 at 1:54

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