A238205 a(n) is the smallest prime such that 2n=p1+p2*p3 where Max(p1,p2,p3)=a(n) and p1, p2 p3 are odd primes.

3, 5, 7, 5, 5, 7, 7, 7, 5, 5, 7, 13, 11, 7, 7, 7, 11, 11, 13, 13, 7, 7, 7, 11, 11, 11, 17, 11, 13, 13, 13, 19, 13, 13, 11, 11, 11, 23, 11, 13, 17, 13, 13, 13, 19, 13, 13, 19, 17, 19, 19, 19, 29, 23, 23, 17, 11, 11, 11, 17, 11, 13, 17, 17, 19, 23, 19, 13, 13, 13

Offset

6,1

Comments

a(n) is defined for integers n >= 6.

Links

Lei Zhou, Table of n, a(n) for n = 6..10000

Example

n=6, 2n=12=3+3*3, Max[3,3,3]=3, so a(6)=3;
n=7, 2n=12=5+3*3, Max[5,3,3]=5, so a(7)=5;
..
n=10, 2n=20=11+3*3=5+5*3,  Max[11,3,3]=11, Max[5,5,3]=5, Min[11,5]=5, so a(10)=5.

Mathematica

Table[p = 2; r = n; While[p = NextPrime[p]; m = n - p; m >= 9, f = Transpose[FactorInteger[m]]; If[(Total[f[[2]]] == 2)&&(f[[1, 1]]>2), s = Max[Max[f[[1]]], p]; If[r > s, r = s]]]; r, {n, 12, 150, 2}]

Crossrefs

Cf. A002375.

Sequence in context: A279321 A254863 A085965 * A186702 A141710 A279399

Adjacent sequences: A238202 A238203 A238204 * A238206 A238207 A238208

Keyword

nonn,easy

Author

Lei Zhou, Feb 20 2014

Status

approved