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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 (list; graph; refs; listen; history; text; internal format)
 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

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Last modified December 4 09:43 EST 2021. Contains 349485 sequences. (Running on oeis4.)