Thursday, 26 February 2015

number theory - Are there infinitely many primes with digit sums of the form n2+1?



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 p1 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 373+7=32+1 . The primes sum of whose digits is of the form n2 (Eg 314=22) or n21 (Eg 718=321) 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:



n21,3,31,79,97,103,211,277,367,349



n213,17,53,71,107,233,251





There is a reason why I called primes whose sum of digits is of the form n2 and n21 Almost nice primes.



If you have an Almost Nice Prime of the form n21 then adding 2×10k to it (here k is the highest power of 10 in decimal expansion of n21) will give you a Nice prime if it is a prime (by it I mean n21+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 n21)+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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