OFFSET
1,4
COMMENTS
Conjecture: (i) a(n) > 0 for all n > 2.
(ii) If n > 30, then 2*n + 1 can be written as 2*p + q with p, q and prime(p + 2) + 2 all prime.
Part (i) implies both the Goldbach conjecture and the twin prime conjecture. If all primes p with prime(p + 2) + 2 are smaller than an even number N > 2, then for any such a prime p the number N! + N - p is in the interval (N!, N! + N) and hence not prime.
Similarly, part (ii) implies both Lemoine's conjecture (cf. A046927) and the twin prime conjecture.
We have verified part (i) of the conjecture for n up to 2*10^8.
LINKS
Zhi-Wei Sun, Table of n, a(n) for n = 1..10000
EXAMPLE
a(10) = 1 since 2*10 = 3 + 17 with 3, 17 and prime(3 + 2) + 2 = 11 + 2 = 13 all prime.
a(589) = 1 since 2*589 = 577 + 601 with 577, 601 and prime(577 + 2) + 2 = 4229 + 2 = 4231 all prime.
MATHEMATICA
p[m_]:=PrimeQ[Prime[m+2]+2]
a[n_]:=Sum[If[p[Prime[k]]&&PrimeQ[2n-Prime[k]], 1, 0], {k, 1, PrimePi[2n-1]}]
Table[a[n], {n, 1, 100}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Zhi-Wei Sun, Jan 28 2014
STATUS
approved