OFFSET
1,11
COMMENTS
Conjecture: a(n)>0 for all n=6,7,...
This has been verified for n up to 10^8.
Zhi-Wei Sun also made the following general conjecture:
Let P(x) be any non-constant integer-valued polynomial with positive leading coefficient. If n is large enough, then there is a prime p<n such that 6P(n)+p and 6P(n)-p are both prime. For example, for P(x)=x(x+1)/2, x^2, x^3, x^4 it suffices to require that n is greater than 1933, 2426, 6772, 24979 respectively.
See also A219023 for similar conjectures.
LINKS
Zhi-Wei Sun, Table of n, a(n) for n = 1..20000
Zhi-Wei Sun, Conjectures involving primes and quadratic forms, arXiv preprint arXiv:1211.1588, 2012.
EXAMPLE
a(11)=2 since the 5 and 7 are the only primes p<11 with 66-p and 66+p both prime.
MATHEMATICA
a[n_]:=a[n]=Sum[If[PrimeQ[6n-Prime[k]]==True&&PrimeQ[6n+Prime[k]]==True, 1, 0], {k, 1, PrimePi[n-1]}]
Do[Print[n, " ", a[n]], {n, 1, 20000}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Zhi-Wei Sun, Nov 10 2012
STATUS
approved