OFFSET
1,5
COMMENTS
Conjecture: for all n > 3, a(n) > 0.
If 2n = p + q and p+6 is also a prime, 2n+6 can be written as the sum of two primes p+6 and q.
The conjecture is weaker than a conjecture of Sun posed in 2012 (see A219055). - Zhi-Wei Sun, Mar 18 2015
LINKS
Lei Zhou, Table of n, a(n) for n = 1..10000
Lei Zhou, Plot for 0<n<=200000.
EXAMPLE
n=4: 2n=8=3+5, 5+6=11 is also a prime number. This is the only occurrence, so a(4)=1.
n=5: 2n=10=3+7=5+5. Both 5+6=11 and 7+6=13 are prime numbers. Two occurrences found, so a(5)=2.
MATHEMATICA
Table[e = 2 n; ct = 0; p1 = 2; While[p1 = NextPrime[p1]; p1 <= n, p2 = e - p1; If[PrimeQ[p2], If[PrimeQ[p1 + 6] || PrimeQ[p2 + 6], ct++]]]; ct, {n, 1, 100}]
PROG
(PARI) a(n) = {nb = 0; forprime(p=2, 2*n, if ((q=2*n-p) && (q <= p) && isprime(q=2*n-p) && (isprime(q+6) || isprime(p+6)), nb++); ); nb; } \\ Michel Marcus, Mar 01 2015
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Lei Zhou, Feb 23 2015
STATUS
approved