OFFSET
0,11
COMMENTS
The graph of this function shows two main branches, each with further subdivisions. It seems that the main branches result from the fact that values a(3k+1) are in the mean roughly 30% lower than values a(3k) and a(3k+2). This can be explained by the fact that the sum of 3 primes (with equal probability of being congruent to 1 or to 5 mod 6) is congruent to 3 (mod 6) in only 2 out of 8 cases, and congruent to 1 or to 5 (mod 6) in 3 out of 8 cases, for each of these two residues. Analyzing the frequencies of the possible residues mod 30 explains the further sub-branches: A sum of 3 primes is congruent to 1, 3, ..., 29 (mod 30) in (42, 29, 33, 39, 29, 36, 36, 30, 39, 30, 39, 30, 36, 37, 27) out of 512 cases. - M. F. Hasler, Oct 27 2017
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..10000
EXAMPLE
a(19) = 6 because 2*19+1 = 39 and 39 = 3+5+31 = 3+7+29 = 3+13+23 = 3+17+19 = 5+11+23 = 7+13+19.
PROG
(PARI) A102605(n, s=0)={forprime(p=1, (n*=2)\3, my(d=n-p); forprime(q=p+1, d\2, isprime(d+1-q)&&s++)); s} \\ M. F. Hasler, Oct 27 2017
CROSSREFS
Number of ways of writing 2n+1 as p+q+r where p, q, r are primes with p <= q <= r gives A054860.
Bisection of A125688 (odd part). - Alois P. Heinz, Nov 14 2012
KEYWORD
nonn,look
AUTHOR
Zak Seidov, Jan 29 2005
STATUS
approved