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A185636 a(n) = |{0 <= k < n: n+k and n+k^2 are both prime}|. 15
0, 2, 2, 2, 1, 2, 3, 1, 2, 3, 2, 3, 3, 1, 4, 2, 2, 3, 2, 2, 2, 5, 3, 3, 5, 2, 5, 6, 3, 4, 3, 3, 4, 4, 3, 5, 9, 3, 6, 5, 2, 6, 5, 3, 5, 3, 4, 5, 3, 4, 3, 4, 4, 4, 7, 3, 9, 14, 2, 8, 2, 4, 8, 6, 4, 3, 8, 2, 5, 9, 4, 7, 5, 2, 6, 4, 6, 12, 6, 4, 4, 7, 4, 8, 8, 3, 6, 8, 4, 8, 8, 5, 11, 4, 6, 5, 11, 7, 12, 10 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Conjecture: a(n) > 0 for all n > 1.

This conjecture has been verified for n up to 10^8. It is stronger than Bertrand's postulate proved by Chebyshev in 1850.

Zhi-Wei Sun also guessed the following refinement of the conjecture: For any integer n > 1456 there is an integer k among 0,...,n-1 such that n-k, n+k and n+k^2 are all prime; in other words there is a prime p <= n such that 2n-p and n+(n-p)^2 are both prime.

For other refinements of the conjecture, the reader may consult arXiv:1211.1588.

The author also conjectured the following polynomial analogs:

(i) For a given integer polynomial f(x) of degree n > 0, there is an integer polynomial g(x) of degree at most n such that f(x)+g(x) and f(x)+g(x)^2 are both irreducible in Z[x].

(ii) Let F be a field with characteristic different from 2 and 3. If f(x) is an irreducible polynomial over F with degree n > 0, then there is a polynomial g(x) over F with deg(g) <= n such that f(x)+g(x)^2 is irreducible over F.

In a 2017 paper, the author announced a USD $100 prize for the first solution to his conjecture that for each n = 1,2,3,... there is an integer k among 0,...,n such that n+k and n+k^2 are both prime. - Zhi-Wei Sun, Dec 03 2017

LINKS

Zhi-Wei Sun, Table of n, a(n) for n = 1..20000

Z.-W. Sun, Problems on combinatorial properties of primes, arXiv:1402.6641 [math.NT], 2014-2015.

Z.-W. Sun, Conjectures on representations involving primes, in: M. Nathanson (ed.), Combinatorial and Additive Number Theory II: CANT, New York, NY, USA, 2015 and 2016, Springer Proc. in Math. & Stat., Vol. 220, Springer, New York, 2017, pp. 279-310. (See also arXiv:1211.1588 [math.NT].)

EXAMPLE

a(14)=1 since 3 is the only k among 0,...,13 with 14+k and 14+k^2 both prime.

MATHEMATICA

a[n_]:=a[n]=Sum[If[PrimeQ[n+k]==True&&PrimeQ[n+k^2]==True, 1, 0], {k, 0, n-1}]

Do[Print[n, " ", a[n]], {n, 1, 100}]

nk[n_]:=Count[Range[0, n-1], _?(And@@PrimeQ[n+{#, #^2}]&)]; Array[nk, 100] (* Harvey P. Dale, Jun 17 2014 *)

CROSSREFS

Cf. A035250.

Sequence in context: A264051 A120965 A151931 * A182597 A290491 A194314

Adjacent sequences:  A185633 A185634 A185635 * A185637 A185638 A185639

KEYWORD

nonn

AUTHOR

Zhi-Wei Sun, Dec 18 2012

STATUS

approved

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Last modified October 16 12:34 EDT 2018. Contains 316263 sequences. (Running on oeis4.)