The motivation for this question is from one of well known Landau Problems which asks for proof of the statement:
Are there infinitely many primes p such that p−1 is a perfect square? In other words: Are there infinitely many primes of the form n2+1.
But this is not what I am asking. Before I ask my question let me explain the scenario.
A prime p is called Nice prime if sum of its digits is of the form n2+1 For example 37→3+7=32+1 . The primes sum of whose digits is of the form n2 (Eg 31→4=22) or n2−1 (Eg 71→8=32−1) will be called Almost Nice primes.
The question is are there infinitely many Nice primes?
Now, I tried to find Nice and Almost Nice primes by hand till 400 and here is what I've got:
Nice primes are:
5,11,19,23,37,41,73,89,101,109,113,127,131,163,179,181,197,271,307,311,359,373.
While Almost Nice primes are:
n2→1,3,31,79,97,103,211,277,367,349
n2−1→3,17,53,71,107,233,251
There is a reason why I called primes whose sum of digits is of the form n2 and n2−1 Almost nice primes.
If you have an Almost Nice Prime of the form n2−1 then adding 2×10k to it (here k is the highest power of 10 in decimal expansion of n2−1) will give you a Nice prime if it is a prime (by it I mean n2−1+2×10k). In a similar manner, if a prime p is an Almost Nice prime of the form n2 then if n2+10k is a prime then it will be a Nice prime.
But introducing Almost Nice prime is not much helpful as we have to make sure that Almost Nice prime (of the form n2)+10k or Almost Nice prime (of the form n2−1)+2×10k is a prime before concluding that it met our Nice prime criteria.
Since the post is long, I again remind you the question. Are there infinitely many Nice primes?
Thanks.
Answer
Even the set of positive integers with digit sum 101, only having the digits 0 and 1 and ending with a 1, contains \binom{n-2}{99} n-digit numbers.
This means, that we have , for example, about \large \color\red {10^{438}} numbers with a million digits in the set. Plenty of them should be primes, since they share no common factor.
If n increases, the binomial coefficient grows much faster than n itself. So there is an overwhelming statistical evidence that infinite many nice primes exist.
Of course, such an evidence is no proof.
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