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 A084834 a(n) is the smallest number not previously used such that a(n)+a(n-1), a(n)+a(n-1)+a(n-2), ..., a(n)+...+a(1) are not prime. 5
 1, 3, 5, 7, 9, 11, 13, 15, 17, 4, 6, 30, 38, 10, 36, 14, 34, 42, 39, 21, 69, 27, 33, 45, 20, 16, 24, 50, 25, 51, 66, 18, 72, 54, 60, 74, 22, 8, 19, 41, 28, 48, 44, 40, 78, 57, 35, 58, 102, 12, 63, 65, 64, 56, 46, 96, 68, 76, 114, 80, 52, 84, 90, 99, 55, 2, 93, 75, 100, 62, 120, 98 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS No sum of a continuous subsequence is ever prime. Is every integer used? Can an odd number ever be surrounded by two even numbers (and similarly for even numbers)? LINKS Chai Wah Wu, Table of n, a(n) for n = 1..10000 EXAMPLE a(4)=7 because 7 is the smallest unused number such that 1+3+5+x, 3+5+x and 5+x are all composite PROG (PARI) checkprime(a, b)=local(fl); fl=0; for (i=1, b-1, if (isprime(a+s[i]), fl=1; break)); if (fl==0, for (j=1, b-1, if (a==p[j], fl=1; break))); fl p=vector(300); p[1]=1; pc=2; while (pc<300, x=1; s=vector(300); for (i=1, pc-1, s[i]=sum(k=i, pc-1, p[k])); i=1; while (checkprime(x, pc), x++); p[pc]=x; pc++); p CROSSREFS Cf. A084833, A254337. Sequence in context: A187232 A187907 A024806 * A118137 A160931 A160924 Adjacent sequences: A084831 A084832 A084833 * A084835 A084836 A084837 KEYWORD nonn AUTHOR Jon Perry, Jun 06 2003 STATUS approved

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Last modified April 17 05:47 EDT 2024. Contains 371756 sequences. (Running on oeis4.)