A302756 a(n) is the least possible greatest prime in any partition of prime(n) into three primes; n >= 4.

3, 5, 5, 7, 7, 11, 11, 13, 13, 17, 17, 17, 19, 23, 23, 29, 29, 31, 31, 31, 31, 37, 41, 37, 41, 41, 41, 43, 47, 47, 53, 53, 61, 61, 61, 61, 61, 61, 61, 71, 67, 71, 71, 73, 79, 83, 79, 83, 83, 83, 89, 89, 97, 97, 101, 97, 101, 97, 103, 103, 107, 107, 107, 113, 127, 127

Offset: 4

Author

David James Sycamore, Apr 12 2018

Keywords

• nonn

Comments

Goldbach's weak (ternary) conjecture states that every odd number > 5 can be expressed as the sum of three primes (see link). This sequence applies the conjecture (now proven) to primes > 5. From all possible partitions of prime(n) = p+q+r for primes p,q,r (p <= q <= r), a(n) is chosen as the least possible value of the greatest prime r (with lower prime p not constrained to be A302607(n)). The sequence is not strictly increasing, and although many primes appear repeatedly, some do not appear at all (e.g. 59 is not included).

Examples

			The partition of prime(5)=11 into 3 primes p <= q <= r is 11=3+3+5 and since no smaller value than 5 can be attributed to r, a(5)=5.
		

Links

• Michel Marcus, Table of n, a(n) for n = 4..5000
• Wikipedia, Goldbach's weak conjecture

Crossrefs

Cf. A024684, A224534, A302607.

Programs

PARI

a(n) = {my(pn = prime(n), res = oo); forprime(p=2, pn, forprime(q=p, pn, forprime(r=q, pn, if (p+q+r == pn, res = min(res, r));););); res;} \\ Michel Marcus, May 13 2018

PARI

first(n) = {n = prime(n + 3); my(strt = vector(n, i, i), t = 0, res = vector(primepi(n) - 3)); forprime(p = 2, n, forprime(q = p, n - p, forprime(r = q, n - p - q, strt[p + q + r] = min(r, strt[p + q + r])))); forprime(p = 7, n, t++; res[t] = strt[p]); res} \\ David A. Corneth, May 14 2018