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A128687
Smallest odd nonprime such that every subset of a(1), ..., a(n) adds to a nonprime.
3
1, 9, 15, 39, 105, 731, 2805, 59535, 2658795, 78060135, 67554224745
OFFSET
1,2
COMMENTS
The first 8 terms are from Rivera's puzzle 84.
The sequences is infinite [Chris Nash]. - N. J. A. Sloane, Jan 20 2017
LINKS
Chris Nash, Proof that A052349, A128687, and A128688 are infinite [Cached copy of proof, from The Prime Puzzles and Problems website]
Carlos Rivera, Puzzle 84. Non-primes adding up to non-primes, The Prime Puzzles and Problems Connection.
MATHEMATICA
a[1]=1; a[n_]:=a[n]=(s=Subsets[Array[a, n-1], n-1]; c=a[n-1]; While[d=1; While[!PrimeQ[Total[s[[d]]]+c]&&d<Length@s, d++]; d!=Length@s||PrimeQ[Total[s[[d]]]+c]||PrimeQ@c, c=c+2]; c); Array[a, 8] (* Giorgos Kalogeropoulos, Nov 19 2021 *)
CROSSREFS
Cf. A052349 (no restrictions on even or odd), A128688 (restricted to even numbers).
Sequence in context: A373333 A100556 A057478 * A193579 A274757 A146789
KEYWORD
hard,nonn
AUTHOR
T. D. Noe, Mar 20 2007
EXTENSIONS
a(11) from Donovan Johnson, Apr 18 2010
STATUS
approved